# [Wet Thumb Forum]-Phosphorus



## Roger Miller (Jun 19, 2004)

A few years ago I posted a four part article on APD about phosphorus. I thought that I'd upgrade the graphics a little and repost it here for a hopefully new audience.

The letters will be posted almost exactly as they were on APD, so the context may seem a little odd.


Roger Miller


----------



## Roger Miller (Jun 19, 2004)

A few years ago I posted a four part article on APD about phosphorus. I thought that I'd upgrade the graphics a little and repost it here for a hopefully new audience.

The letters will be posted almost exactly as they were on APD, so the context may seem a little odd.


Roger Miller


----------



## Roger Miller (Jun 19, 2004)

Folks:

Recently I spent some time pondering the role of phosphorus in aquariums.

We usually think of phosphorus more in terms of dissolved phosphate, but (excepting the many biological forms) phosphorus actually occurs in aquariums in four different forms: dissolved inorganic phosphorus, particulate inorganic phosphorus, dissolved organic phosphorus and particulate organic phosphorus. The inorganic forms are phosphates and the organic forms are molecules derived from the decomposition of biochemicals. There are important interconversions between the different forms.

The chart that follows illustrates some f the more important aspects of the system.









The sketch shows each of the four major types of phosphorus, the major sources for each type, the interconversions between them and the possible fates of each form. For simplicity, biological uptake of phosphorus isn't included in the sketch. The pathways shown with heavy lines are pathways that might be promoted to reduce problems with dissolved phosphorus levels in an aquarium. The pathways shown with light lines are pathways that might be supressed to reduce phosphorus problems.

This whole thing could be given a lot of words, but I'm going to try to limit myself to a brief description.

*Dissolved inorganic phoshorus* is biologically available and essential to plants and algae. It is mostly dissolved phosphates. It enters the aquarium from tap water, sometimes from water treatments like pH buffers and as inorganic phosphorus wasted through the gills and kidneys of animals. It is removed from the aquarium by water changes. It is produced in aquariums from organic phosphorus by bacterial and algal phosphatase activity. It is converted to particulate inorganic phosphorus by sorption and precipitation.

*Particulate inorganic phoshorus* is mostly not biologically available. It is phosphate associated with phosphate minerals and adsorbed on metal hydroxides and other solids in the aquarium. It enters the aquarium mostly in fish food and animal feces and it can be removed by siphoning out detritus and cleaning filter media. It is formed within an aquarium by sorption and precipitation of dissolved inorganic phosphorus.

*Dissolved organic phosphorus* is biologically available to bacteria and possibly to algae, but I haven't found references saying it is available to plants. It enters the aquarium from animal waste. It can be removed by water changes. Bacterial and algal phosphatase activity convert dissolved organic phosphorus to phosphates.

*Particulate organic phosphorus* isn't available to plants and algae, but is available to animals. It enters the aquarium as plant detritus, fish food and feces. It can be removed by siphoning and filter cleaning. It is converted to dissolved phosphates by phosphatase activity. The phosphatase activity is partly due to detritivores but also to bacteria and possibly algae.

I wrote a little spreadsheet program to balance the reactions and provide estimates for steady-state concentrations of each component with various estimates of the reaction rates. If there is interest then in later letters I can try to summarize what that all seems to mean to controlling phosphorus levels in an aquarium.

Roger Miller


----------



## Roger Miller (Jun 19, 2004)

My goal is to help people find ways to control excessive algae growth caused by high phosphorus levels. It should be possible to control those problems with good tank-management habits -- without large investments in chemical test kits and a minor in chemistry.

Yesterday, to that end I foisted this diagram on the list:









The most evident controls on all the phosphorus types is to limit the sources (fish food, fish wastes, and phosphates in tap water) and increase removal with larger or more frequent water changes and more effective removal of phosphorus-containing sediments.

*Water changes* seem like a great way to keep dissolved forms of phosphorus from building up in a tank, but that may not be as effective as it seems at first blush. In my case, for instance, I do 15% water changes in every tank, every week. Along with 15% of the water, the water changes remove only 15% of the phosphorus. Commonly recommended change schedules like 20-25% every two weeks (equivalent to 10% to 12.5% per week) are even less effective. Still, 10% is better than nothing and for most of us practical limits on time and energy keep our water changes at this sort of low level.

People who have significant phosphate in their water supplies have a different problem. For them, water changes may actually increase rather than decrease the phosphorus supply in their aquarium. For those people small water changes may be better. They may need to use water run through a commercial phosphate-removing media like activated alumina, or purified (ultrafiltered or deionized) water.

*Sediment removal* - usually done just by siphoning off detritus and cleaning filters - is another great-sounding ploy, but here too there are some snags. In a bare tank it's easy to remove all the visible detritus during the periodic cleanings and to wash all of the built up material from filters. It's a lot harder to get the detritus out of a heavily planted tank, but even with heavy planting it should be possible to remove more than 50% of the detritus at each cleaning.

Most of us clean our tanks and filters on a periodic basis. Between cleanings the accumulated organic particulates can be attacked by bacteria and release dissolved phosphorus. By the time we remove the accumulated particulates we may be getting only a small fraction of the phosphorus that it originally contained.

On general principles filters should be cleaned as frequently as possible to limit the release of dissolved phosphorus. Unfortunately most of the popular aquarium filters are difficult enough to get into that daily cleanings are out of the question. For most of us, daily siphoning of detritus is also out of the question.

Both cleaning and water changes provide important controls on the accumulation of phosphorus in the aquarium but they are of limited effectiveness. There are a few additional options for phoshorus removal that depend on conversions within the aquarium and I'll describe those when I get around to writing about those conversions.

That brings us to the options for controlling the additions of phosphorus to our aquariums. It's pretty easy to avoid adding phosphate fertilizers or other phosphorus-containing chemicals to the tank. It's more difficult, but possible, to avoid adding phosphorus to the tank in the water supply that's used for water changes. Once those inputs are controlled, we're still left with a large influx of phosphorus. That influx is from feeding, either directly as uneaten food or indirectly as feces.

*Feeding* can be controlled in two different ways; in quantity and in quality. 
Controlling the quantity of feedings is pretty straightforward. Many of us (including myself) probably feed more than is necessary for maintaining healthy adult fish. Since feeding is the major source of phosphorus for the tank, reducing the feeding can have a proportionate effect on the phosphorus level in the tank. Decreasing the feeding rate by 30% can reduce the phosphate level in the tank by 30%, a 50% decrease in feeding can halve the phosphate levels, and so on. Most of us are probably pretty happy with the way we're feeding our fish and may not be comfortable with large reductions, so there's a limit to the control that can be attained this way. Probably the most comfortable way to reduce feedings is to reduce the fish population in a planted tank. That lets you reduce the feeding without running the risk of stressing your favorite fish.

Controlling the quality of the feed is not as straightforward. The main idea is to use food with a lower phosphorus content. Many prepared foods contain more phosphorus than fish need and not all (perhaps less than half) of the phosphorus is even in a form the fish can use. Look for foods with lower phosphorus content that are formulated for the specific needs of your fish. You can even do this with live and fresh foods if you can find the phosphorus content of the foods you're using.

There is also an approach for those who want to force phosphorus to be more growth-limiting than nitrogen. Feed your fish a diet with a ratio of at least 7:1 between available nitrogen content and available phosphorus content. At that ratio, phosphorus should be in short supply compared to nitrogen. It's fairly easy to check this with prepared foods. The total nitrogen content can be estimated by dividing the crude protein content by 6.25. Probably most of the nitrogen content will be biologically available. Total phosphorus content should be listed on the packaging. As much as half the phosphorus content may be biologically unavailable, but it's safer to assume (at least initially) that all of it will be biologically available. If you use a diet that is 45% protein (for instance), then it will contain 45/6.25=7.2% nitrogen. In order for phosphorus to be growth-limiting the phosphorus content of the food should be about 1/7th that amount, or 1% of the total. If the actual phosphorus content of the diet is 1% or less, then phosphorus should be growth limiting. This is not as easy when using live or fresh foods but as long as you can find the protein and phophorus contents of the food you can use the method.

***

Hopefully that answered some of the initial questions left after my first letter. In another letter I'll try to describe some of the interconversions between forms of phosphorus in an aquarium. I'll even give thought to how they might be used to control phosphate levels in an aquarium.

Roger Miller


----------



## Roger Miller (Jun 19, 2004)

Folks,

Remember this?









Two days ago I described the forms of phosphorus shown in the diagram and listed the mechanisms that add and remove each form. Yesterday I described the major routes in and out of the aquarium and some means of using those routes to control the phosphorus levels. Today I guess I'll write about the reactions within the aquarium and how they might be used to help control the concentration of dissolved forms of phosphorus.

The terms for different forms of phosphorus are repeated frequently in this discussion. I'm going to use some abbreviations to keep the text from becoming far too cumbersome:

DPi is dissolved inorganic phosphorus
PPi is particulate inorganic phosphorus
DPo is dissolved organic phosphorus
PPo is particulate organic phosphorus

*Phosphatase* is an enzyme that bacteria use to convert DPo and PPo to DPi. Some algae may also have that ability. Alkaline phosphatase - which works when the pH is near and above 7 - is the most well documented form of phosphatase. There is also an acid phosphatase that doesn't seem to be as well documented. Phosphatase activity increases with the size of the bacterial population; activity is promoted by oxygen and suppressed by dissolved phosphate.

Bacteria typically produce more phosphate than they consume, and that can lead to an excess of phosphate in the water.

The phosphatase-catalyzed reaction from PPo to DPi should be discouraged to prevent production of excess dissolved phosphate. If the reaction can be slowed down then phosphorus will remain in the detritus where it can be more easily siphoned off before it causes problems.

The reaction from DPo to DPi is a somewhat different matter. My reading indicates that some algae may be able to utilize DPo but plants cannot. If that's true then DPi - which can be utilized by plants and algae - are preferred to DPo, so the conversion of DPo to DPi should be encouraged.

How do we suppress the phosphatase-catalysed reaction from PPo to DPi while we still allow the phosphatase catalyzed reaction from DPo to DPi? I don't know, but algal scrubbers come to mind. My impression is that this is probably a fairly fast conversion that would be difficult to influence.

I can speculate about how we might reduce the rate of phosphatase-catalyzed reactions, but I don't have any real data. The conditions that promote high levels of phosphatase activity and so cause rapid release of phosphorus from detritus are exactly the conditions found in aquarium filters. In a filter, organic particulates are trapped in a constant flow of aerated water where a large population of bacteria can act on the detritus. The flowing water also flushes the phosphate released by the reaction, which otherwise could suppress further phosphatase activity. So removing a filter should reduce the phosphatase activity.

I keep unfiltered tanks. I think in retrospect that pulling the filters off my tanks helped control phosphate levels, but I can't substantiate that. Has anyone measured phosphates over a period of time after removing the filter from a planted tank? Or observed changes in a tank after removing the filter that would suggest a change in phosphate levels?

*Precipitation and sorption* convert DPi to biologically unavailable PPi. This is a group of reactions that in nature are largely responsible for keeping phosphorus in a growth-limiting role.

DPi combines with calcium to precipitate a number of insoluble solids. Dissolved calcium (hardness) and pH near and above 7 promote formation of the solids. The solids don't precipitate readily and may not form at all without a suitable substrate or "seed" to grow on. Industrial and municipal-scale phosphorus removal systems use several different kinds of seeds. One system uses plain sand as a seed for forming the phosphorus-bearing solids, other processes use calcium carbonate (which isn't stable under common conditions in planted tanks) or one of several kinds of calcium phosphates. Dissolved organic compounds like tannins can foul the seed surfaces and slow down the precipitation of calcium phosphates.

Very soft water and water with a low pH should be prone to elevated phosphate levels because the DPi won't precipitate. If you have excess phosphate in a tank with soft water or pH below 7 then you might be able to increase the calcium content and pH of your water and add a fresh seed material to your filter media to promote precipitation of calcium phosphate solids.

Dissolved phosphates also have a strong tendency to stick to many kinds of surfaces. The generic term for this process is sorption. Phosphorus is very stongly associated with surfaces on ferric oxyhydroxides and aluminum hydroxides. Ferric oxyhydroxides are common in nature. In most soils - including aquatic soils - it is often sorption on ferric compounds that keeps phosphorus in place and out of the water.

Low levels of dissolved phosphorus can be removed from water by dosing with a ferrous chloride solution. The ferrous iron oxidizes to ferric iron and precipitates as the oxyhydroxides. DPi is strongly associated with the iron and settles out with it. That process also lowers pH and alkalinity. DPi may also attach to laterite and other soil substrates that contain ferric oxyhydroxides.

Several companies sell filter materials on the aquarium market that use the same mechanism to pull phosphate out of the water. I think that those products use activated aluminum hydroxide. In my experience, the products work, but often not for long and they can be fairly expensive to use on a regular basis. They attach a lot of compounds other than phosphate. Those other compounds (particularly silica and some organics) can pretty quickly foul the material and ruin it's capacity to remove phosphate. I suspect that phosphate-adsorbing products would be more effective when used as a pre-treatment to remove phosphorus from tap water than they are as an aquarium filter medium.

***

In my last letter on this topic I'll try to summarize all this and use the results of a simple spreadsheet model to assess the relative effectiveness of different control methods.

Roger Miller


----------



## Roger Miller (Jun 19, 2004)

Folks,

Tests with a simple model of phosphorus states in aquariums show that reducing feeding rates is the most effective means of reducing phosphate concentrations. The second most effective ploy is to encourage higher rates of phosphate precipitation or sorption and the third most effective ploy is to increase water changes.

Here it is the familiar diagram again, this time for the last installment:









In my three previous letters I described the major elements in this diagram, discussed the terms that add and remove phosphorus from the system, and most recently described the interactions between the forms of phosphorus within the aquarium. In this last letter I hope to present a simple model of the system and describe the model's results pertinent to controlling dissolved phosphorus levels in aquariums.

Very briefly, the model consists of four equations, each representing the mass balance for each of the four forms of phosphorus in the system. The arrowed lines in the diagram above are represented in the model by a term in one or two of the equations. The resulting system of equations can be solved for the concentration of each of the four phosphorus forms as a function of the input of fish food and fish wastes, with the terms in the equation representing the rates of water changes, sediment removal, phosphatase activity and precipitation or sorption.

Many of the coefficients in the model depend on details of an aquarium setup; maintenance, food composition, species selection and so on. Some of the details are fairly insignificant, and others have duplicative effects. The model has 10 coefficients. I varied two of the coefficients to create three different test cases that cover a range of possible aquarium conditions, Here are all 10 of the model coefficients, and the values used for each of the three test cases.

case 1 case 2 case 3

Feeding rate
grams phosphorus/week 
0.02 same same

Phosphorus digestibility
percent 
50% same same

proportion of inorganic phosphorus
in undigestible fraction 
80% same same

proportion of inorganic phosphorus
in digestible fraction
20% same same

precipitation rate for
dissolved inorganic phosphorus
percent per week
67% 100% 25%

mineralization rate for
dissolved organic phosphorus
percent per week
100% same same

phosphatase activity for
particulate organic phosphorus
percent per week
50% 10% 67%

water change rate
percent per week
15% same same

cleaning efficiency
percent per week
67% same same

resulting phosphate
concentration in a
55 gallon tank
0.21 ppm 0.12 ppm 0.42 ppm

I used the cases to test 5 coefficients to see which ones could be used to most effectively control phosphate concentrations. I measured the effectiveness by calculating the ratio between the percent change in the phosphate concentration and the percent change in the coefficient. Here are the results for each model and each tested coefficient:

case 1 case 2 case 3
feeding rate 
1.0 1.0 1.0

precipitation rate 
-.8073 -.8622 -.6098

water change rate 
-.2713 -.2261 -.4540

cleaning efficiency 
-.0496 -.0243 -.0499

phosphatase activity
for particulate organic phosphorus 
.0494 .0242 .0497

There aren't very many suprises here. In all cases, reducing the feeding rate is the most effective way to decrease phosphate concentrations. The second most effective alternative is to increase the rate of precipitation or sorption of dissolved phosphate. It's interesting that among the three cases, precipitation is less effective for case 3, where the phosphate concentration is highest. Increasing water changes was the third most effective ploy. Again, case 3 is interesting because in the case with the highest phosphate concentration water changes are relatively more effective.

The effectiveness of changing cleaning efficiency and phosphatase activity were substantially lower than for the first three tactics. Oddly, in every case improvements in the cleaning efficiency and decreases in the phosphatase activity had nearly identical effects.


----------



## Roger Miller (Jun 19, 2004)

In retrospect, the emphasis in these articles on reducing phosphorus levels to control algae seems misplaced. The emphasis came from the then widespread use of Sears and Conlin's method (PMDD) for algae control. Their method asserts that nuisance algae should be controlled by fertilizing a tank with everything but phosphorus, thus driving phosphorus down to unmeasurable levels. I don't think that the method is in such widespread use as it was at the time.


Roger Miller


----------



## imported_Allen (Feb 14, 2003)

Roger,

I was just wondering, when we test for PO4, do the test register the level of phosphates for all 4 types? Or just the dissolved inorganics? or something else?

Allen 
============
Allen's Tank Pics.
============


----------



## Roger Miller (Jun 19, 2004)

Allen,

That has been an issue in the technical literature. I think the analyses that we call phosphate test are now commonly called SRP tests in the literature. SRP stands for soluble reactive phosphorus. The tests react mostly to phosphate but will pick up other phosphorus sources that react to the test. I think that will include at least some of the dissolved organic phosphorus.

Exactly what does react to the test depends on the test conditions. The test conditions vary from kit to kit and probably from user to user. It's really hard to say how much non-phosphate phosphorus might be picked up in a given test. This is one reason why phosphate is considered to be a difficult analysis.


Roger Miller


----------



## wetmanNY (Feb 1, 2003)

Allen, though Roger Miller is right to say essentially "it depends" your hunch is on target. If total phosphorus is being measured, all forms of phosphorus are converted to dissolved orthophosphate with acid, persulfate, and heat. A chemical is then added to the water sample. Without such elaborate additional procedures to mineralize organic phosphates, the familiar PO4 test is registering only dissolved inorganic PO4. Since bacteria are constantly mineralizing organics, releasing their PO4 groups, and since precipitated inorganic phosphate is so easily freed and rebound, hobbyist-level phosphate tests are generally more misleading than useful.

[This message was edited by wetmanNY on Thu February 27 2003 at 08:16 AM.]


----------



## Guest (Feb 27, 2003)

The old water changes plus addition of the ortho inorganic phosphate, that's going to benefit the plants best. I use water changes to remove the plant/organic loading in the tank and/or at least from it ever building up very much. Same can be said for DOC/POC.

Although some references suggest substrate based PO4 source are best for plants, I don't observe this in my tanks. I made PO4 balls in the past and put these near the roots and gave them 4 weeks vs adding it to the water column.

Repsonse time from water additions vs root uptake clearly shows direct up take by the leaves. 

I think many times that test kits measure around 0.3ppm or less and the PO4 levels are static, this is from the organic or bound/unavailable PO4.
So it's not helping the plants and may as well be at zero. Those test kits don't always tell what it is your are looking for.

I feel going beyond the range of 0.3ppm etc to be on the safe side is a wise idea. 
If you measure 0.0ppm to start with, then you know adding more via KH2PO4 etc will be primarily the usable form.

I think many folks in the past were "fooled" by the little bit left as bound PO4 and assumed things were okay etc.

Regards, 
Tom Barr 



Regards, 
Tom Barr


Regards, 
Tom Barr


----------



## Roger Miller (Jun 19, 2004)

While reading up on the standard methods for phosphate analysis I came across two tidbits I thought were very interesting.

In older methods for total phosphorus analysis the phosphate was stripped out of organic compounds by a extended hydrolysis step using boiling sulfuric acid. More modern techniques use UV light exposure to do the same thing. I have to wonder if UV sterilizers free phosphate from dissolved organics.

The USGS standard methods provide a detailed description of the phosphomolybdate analysis. I think the same method is used with variations in wet kits. In it the USGS says that it takes 6-10 minutes for the blue color to fully develop and that it degrades afterwards. I thought this confirmed the "slow" test method that I found using the Seachem phosphate kit. Did any of you guys or gals try that technique?

Roger Miller


----------



## Guest (Mar 1, 2003)

The UV comment about PO4 may very well have a more profound effect IMO.
I have used UV's for a long time and used them in conjuction with traces etc and some observations might lead me to say the observations are PO4 linked perhaps. 
Hard say what it is, but it certainly may play a significant role. 

Regards, 
Tom Barr


----------



## wetmanNY (Feb 1, 2003)

How does the tester who is using ultraviolet light to hydrolise organic PO4 know when the sample's phosphate has been entirely converted to orthophosphate and quantitative testing may begin?

If this UV technique is being adopted, it must be described on the Internet somewhere? May we have a look?

A good description of the more familiar testing procedure for total PO4,involving boiling with sulfuric acid for 30 minutes, topping up the sample with distilled water to keep it from boiling dry, is at http://secchi.hmsc.orst.edu/coastnet/manual/phosphates.html

[This message was edited by wetmanNY on Sat March 01 2003 at 01:42 PM.]


----------



## Roger Miller (Jun 19, 2004)

> quote:
> 
> How does the tester who is using ultraviolet light to hydrolise organic PO4 know when the sample's phosphate has been entirely converted to orthophosphate and quantitative testing may begin?


They know how much time it should take and they run standards to make sure it's working right.



> quote:
> 
> If this UV technique is being adopted, it must be described on the Internet somewhere? May we have a look?


The method is used in automated analysis where things like boiling for a half hour are usually avoided. One description of the technique is at:
http://hahana.soest.hawaii.edu/hot/protocols/chap8.html

Roger Miller


----------



## wetmanNY (Feb 1, 2003)

Thank you for that document, Roger Miller. Several readings of it lay open, even to me, the procedure for analysis of seawater phosphate, both organic and orthophosphate. Just the described procedures for avoiding contamination of the samples might alert us amateurs when we're testing for P (at our much higher levels, of course).

This technology detects concentrations on the order of 0.1 _nanomoles_. That would be considered quite P-deficient by AquaBotanic members!

Your post "They know how much time it should take" was less than satisfying to me, because it contains no more information than the statement "They know what they're doing." So I was gratified to read that the irradiation with ultraviolet light is independent of P levels or of time: sufficient time (two hours) is simply allowed to insure that _all_ P is released from any organic matter by photo-oxidation. Aha! now I see.

And I read that periodic calibration checks of the UV lamp efficiency are made using prepared DOC standard solutions. That explains your "phrase "they run standards" which-- though reassuring-- was opaque to an amateur.

So! advanced aquarists are soon to leave behind the boiling sulfuric acid when testing for total phosphate, in favor of this technology, you feel? That is its relevance here, is it not?

[This message was edited by wetmanNY on Sun March 02 2003 at 09:45 AM.]


----------



## Roger Miller (Jun 19, 2004)

The technology developed to analyse tiny amounts of solutes in sea water -- especially the technology required to avoid contamination -- generated a revolution when it was applied to fresh water. It turned out that what we thought we knew about the occurence of many metals in fresh water (copper, lead, zinc, mercury, especially) was a manifestation of poor sampling technique. Now, any freshwater metal analysis more than about 15 years old is regarded as probably incorrect.

I was giving you the reference, so I didn't see much point into providing a lot of detail about the technique.

Do you suppose there are really advanced aquarists out there boiling sulfuric acid for their phosphate analysis?


Roger Miller


----------

