# The Fish as the source of fertilizer for plants



## Cawafuoshi (Mar 24, 2021)

Forgive me if this question appears random, far fetched or outright stupid. But I am wondering, if the aim of the natural game in an aquarium is to rely solely on the soil and whatever waste and CO2 is produced mainly by fish and to a lesser extent by decaying matter, in a densely planted tank, how much fish per gallon is needed (and thus inverting the old fish per gallon rule to limit toxics) to sufficiently nourish all plants?

Or, what do Walstad tank keepers do when they notice deficiencies in plants?


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## mistergreen (Mar 3, 2007)

Whenever I overstock my tanks with fish, they get sick. I add root tabs or dose nutrients into the tank when I see deficiencies. Do what works for your tank. I don't think there are any hard rules.


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## johnwesley0 (Feb 23, 2021)

My nitrate level has been falling steadily since I added artificial lighting to my set up, so I assume there's an inverse relationship between plant growth and nitrates. If the water parameters still indicate the presence of trace amounts of nitrates, there's probably enough bio load to supply both plants and beneficial bacteria with nutrients. Does that make sense?


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## Hank Junior (Oct 1, 2020)

Everyone has their methods but I am increasingly of the opinion that fish and fish food are not required to maintain healthy plants. I have a mini-aquarium right now, a type of grass in soil and natural direct sunlight 2-3 hrs per day, no fish, no snails, only daphnia and other fauna that came with the water I guess, which I only ever top up the water with rain water periodically. I am hoping to replicate it again as I normally struggle to get plants healthy. This tank seems to be on autopilot and needs me not at all.

As I don't keep fish I have always wondered about it, but am pretty convinced that if you can supply the right fertiliser somehow (and maybe top it up periodically) to a planted tank fish are not required.

As far as CO2 goes I don't presently use CO2 injection and yesterday noticed an incredible thing. I am used to my plants pearling a bit at the end of the light day but for 6 months now have noticed that they bubble alot the day I do a partial water change on one of my tanks (50% water change once a week as I use liquid fertilisers daily). I have always assumed that there is some chemical in my water source which my plants always like which tends to only last about a day, the day I do the water change. Yesterday as an experiment I noticed that my plants were not pearling at all and proceeded to add every chemical fertiliser I have on hand, one by one over a period of one hour. I even added a minute amount of KH/GH builder I have to see what would happen (some water salts). Normally I find adding liquid ferts gives a reaction immediately with some pearling but this time nothing, maybe one or two bubbles. Giving up (unable to work out what the water has which my chemical routine doesn't have?) I decided to clean/remove manually the bacteria scum I get a build up of on the water surface. I noticed immediate bubbling of my plants! Incredible. I would never have thought that the gas/water interchange could be so critical to an aquarium.

I know now that the mysterious chemical-X I have been observing for years in my tanks on water change days is in fact CO2 not a nutrient chemical at all. Seems that the water we supply our tanks with is rich in dissolved gases - more gases than we have in the aquarium. Call me stupid - kindof feel stupid now as it seems pretty obvious to me now with hindsight. What wasn't obvious to me was how potentially gas deprived our aquaria can be. A tank full of healthy plants always looked good to me but is potentially a gas-trap! - lacking CO2 which slowly asphyxiates your plants and results in a lack of O2 production which will murder your o2 breathing water fauna.

After yesterday's experience I am thinking that there is plenty of naturally dissolved CO2 in water. As I don't use water pumps or filters (my tanks are still water like ponds) I guess I am more susceptible to a scum layer interfering with gas exchange - a good reason maybe to have some water movement. Today I reinstalled my Eheim water skimmer (water surface cleaner) and will run it daily for 10 minutes like I used to do to keep the surface clean.

Anyways I am not a big believer in CO2 sources for underwater plants any more (I used to believe that) since the effects of atmospheric CO2 gas exchange I witnessed yesterday convinced me that the best source of CO2 for plants is atmospheric. Heres my two bobs worth. Its a good question about fish/fish food and the Walstad tank, I used to wonder about it myself.


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## dwalstad (Apr 14, 2006)

There's more than one way to provide plants with nutrients. It can be done artificially as you are doing or naturally with fish.


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## Hank Junior (Oct 1, 2020)




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## Hank Junior (Oct 1, 2020)

In answer to the original post I think that that is exactly the purpose of the 'natural tank', as you have stated. Above is a photo of the tank that completely changed the way I approach growing plants underwater. Actually it is prototype B. Prototype A was a soil tank I set up decades ago which had a healthy population of Ludwigia Repens growing out of it like in a marsh.

On the left hand side for comparison is an Amano type tank using Amazonia 2 soil and seachem liquid fertilisers (possibly one or two flourish tabs as well - cant remember), Lilaeopsis brasil and hair grass. Sorry about the reflection, its the morning sun. The Amano LHS tank is a looking a bit paler than it was in summer - probably the colder water and the fact I hardly fertilise it any longer but is doing quite well. Both tanks are essentially self-supporting now - no fish, only micro-fauna. Only sunlight and no heating - I just top up evaporation with rain water. You can't see it but both have alot of microfauna including daphnia.

The tank on the RHS has a haze due to the fact that the soil has a very high clay content. Scientists would have probably termed this soil hundred years ago as brown soil (laterite most likely which as an ancient form of clay) - but now its pretty complicated. I have a chart of local soils in my area published by the Government but don't really understand it. Only a chemical analysis would determine what exactly it is. All I know is its what that plant needs on the right. Have tried to grow the grass on the right in other soils but so far no go. It survives Amazonia and seachem ferts but doesn't really grow much, not like in that tank on the right where it grows like a weed. I have no idea what that grass is - found it by the side of a road in a ditch. Only a professional botanist would now. Tank on the right has never seen artificial fertiliser or CO2 injection or anything else added and began with about 8 blades of grass. I had a much more beautiful tank than tank B but unfortunately it met a disaster and didn't survive - its now just a pile of mud. I hope to recreate it soon.

EDIT: correction daphnia seem to have dissapeared, maybe they will return after winter and the water gets warmer? or maybe the lack of liquid fertisiler in the Amano tank has done them in?

By the way Tank B on the RHS may not look beautiful in this dark picture because of its cloudy water but trust me if you saw it in sunlight fully lit - its gorgeous, far more beautiful than the tank on the left. The water has a blue-haze tint to it almost like a sky colour. The Amano tank is easier to see on this picture because of its crystal clear water.


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## mistergreen (Mar 3, 2007)

Hank Junior said:


> The tank on the RHS has a haze due to the fact that the soil has a very high clay content


Why not cap the mud with sand so it doesn't go all over the place?
The mud is probably termed as 'loam', a clay loam. It's a mixture of clay, sand, minerals, and organics.

Daphnia population will crash for a variety of reasons; temperature, pH, food, etc...


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## Hank Junior (Oct 1, 2020)

Yes the reason I don't cap the mud is where I vary from Walstad. May do it one day, but reading her book now has explained to me alot of the things I didn't understand about submerged soils in aquaria.

My original soil tank in the mid-90's used a common black coloured garden soil and had crystal clear water. After reading Walstad's book I now understand how that was possible. I never kept fish in it, was worried about its toxicity, but after reading this book I realise I needn't have worried about that and I expect my fish would have been OK from any toxicity concerns - I used to be able to kill fish quite easily without soil - poor little things - wasn't always a successful fish keeper!

This black soil/mud tank seemed to be surprisingly hard to muddy up the water - the mud would rise up a bit and then quickly settle. For the first two weeks the tank was so muddy you could barely see what was in it. That is the reason I don't cap my soil - its part of the experiment. If I ever acheive a plant tank large enough to keep a fish in it I will soon find out. Another curious thing I have noticed about fish (not bottom dwellers). I have noticed that most fish rarely contact the surface of the substrate. Probably because of their slime coats they avoid contact with aquarium objects. I suspect that a small fish in a non-capped mud tank will hardly muddy up the base at all. One day I will find out. A large fish that borrows into the substrate would be a disaster in a mud tank I would expect.


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## Hank Junior (Oct 1, 2020)

RE: SOIL TYPES. yes I have been meaning to ask our neighbours who are farmers about the local soils, they are farmers (we only keep stock, no farming). I expect they have their own definitions. Often when you see a property for sale in central NSW they will often describe a variety of soil types with different colours. The CSIRO (which is our local government science authority) has terms I suspect the farmers don't use. From what I have seen it is extremely complicated, at least the CSIRO government publications. They also sometimes describe the ancient geology of the area with names like 'Carrabear formation'- 'Quartenary-age' whatever that means?, which may be a different classification to the soil type?, more to do with ancient geology than its soil properties (chemical analysis)? Don't know. Kurosols, Sodosols.... it makes your head ache... I hope my neighbours have an easier definition, and I expect your description is closer to how they would describe their soils with maybe a colour added for differentiation? My one observation of this soil which was taken from the same spot as the plant is its more yellow than the soil I have on my place which tends to be more orange when submerged - but maybe the yellow colour will come with time? My submerged soils (soil taken from my farm) seem to be more orange to grey on the colour scale. Looks more orange when you first submerge it, but all the farm dam muds are more grey in colour though the water colour is often very orange-brown., and the farm dam muds must be the same soil.


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## Cawafuoshi (Mar 24, 2021)

dwalstad said:


> There's more than one way to provide plants with nutrients. It can be done artificially as you are doing or naturally with fish.


Could we come up with a general rule of thumb how many inches of fish per gallon are required to provide sufficient nutrients from this source in a "typical" Walstad tank? And, if that is not practical for reasons of overstocking, etc., what artificial fertilizers are recommended?


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## johnwesley0 (Feb 23, 2021)

Cawafuoshi said:


> Could we come up with a general rule of thumb how many inches of fish per gallon are required to provide sufficient nutrients from this source in a "typical" Walstad tank? And, if that is not practical for reasons of overstocking, etc., what artificial fertilizers are recommended?


I'm as curious as you are about whether such a rule of thumb exists. The truth is the question rarely comes up and I think it's because plants can generally tolerate higher levels of ammonia than fish can. Even with one fish in a closed space, the concern is always that the fish will poop more than can be converted to harmless amino acids (by plants) or less harmful nitrates (by bacteria.)


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## Cawafuoshi (Mar 24, 2021)

It's just a thought that crossed my mind some days ago, and I apologized from the get go if this question was in one way or the other unfounded. Probably I misunderstand something about fish keeping in general or building a successful Walstad tank.

Some weeks ago I concluded by reading posts and asking my own questions that many Walstad tanks seem to be "overstocked" if we consider traditional advice about fish keeping. On the other hand, many fish keepers seem to get away with it because of their heavily planted, and I assume, healthy tanks.

Obviously, more fish lead to higher concentrations of bio load which, from my understanding, ought to benefit plant growth. But in tanks with low bio load, would you expect less/slower plant growth because of it? Could a too low bio load even cause deficiencies in nutrients? Have you had tanks with high bio load that were consistently more successful than tanks with a low one?

Also, I'm curious what Walstad enthusiasts usually do when they observe nutrient deficiencies in their plants. Add fertilizer or increase bio load by getting more fish or snails?

I understand, deficiencies also could come from an inappropriate light source, but Walstad tanks usually get some exposure to natural day light that should help with growth.


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## johnwesley0 (Feb 23, 2021)

There are so many aspects to this question and as a novice, I can only think of them one at a time! For example, I just remembered a little bit of a thread (couldn't even tell you the name of it) that repeated something Ms. Walstad had written regarding how _rooted_ plants differ from the way _stem_ plants absorb nutrients. Stem plants (if I'm recalling it correctly) are going to be much more dependent on nutrients in the water column because they are absorbed through their leaves; rooted plants on whatever's in the soil. Obviously, if your substrate runs out of decaying organic matter or whatever fertilizer came with the potting mix (as apparently happens more often than we think) you have no choice but to resort to various additions to the soil including bone meal, potassium sulfate (KCl usually in the form of a pinch of kitchen salt substitute.) For a lot of people (myself included) it's a matter of a couple of root tabs now and then.


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## Cawafuoshi (Mar 24, 2021)

"Father Fish" on YT claims he has solved the problem of the soil running out of nutrients. He claims some of his 10 years old tanks are running better nowadays than when they were new, and he follows the Walstad method with his own additions to the soil when setting up a tank. I haven't looked into this yet.

Yes, the water column is not the only distribution channel. I would like to think that over time the increasing amount of mulm will contribute to rooted plants' growth too. So, the fish waste gets distributed via different forms of uptake by plants. That leads me back to my original question: can we say, how many inches of fish per gallon will benefit plants the most? Probably we can't and that is fine. I feel the question needed to be raised nevertheless. 

And I am also okay with using fertilizers to address deficiencies, but I can imagine that is exactly what Walstad tank keepers would do only as a last means of addressing the problem. Hence the many "No Co2, no ferts" projects on YT and the forums.


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## johnwesley0 (Feb 23, 2021)

Cawafuoshi said:


> Yes, the water column is not the only distribution channel. I would like to think that over time the increasing amount of mulm will contribute to rooted plants' growth too. So, the fish waste gets distributed via different forms of uptake by plants. That leads me back to my original question: can we say, how many inches of fish per gallon will benefit plants the most? Probably we can't and that is fine. I feel the question needed to be raised nevertheless.


I think over time, the biggest contributor to plant growth in the soil is probably slowly decaying organic matter, the stuff that remains after you've removed the biggest pieces of bark floating to the top of your pail during the "mineralizing" process (which is a misnomer.) It releases CO2 into the substrate; it provides matter for bacteria to break down and it probably outlasts the fertilizer that gets added to many mixes sold over the counter. You're right: Somewhere there's a Golden Mean between overstocking your tank with _fish_ and thereby poisoning them with their own poop or overstocking the tank with _plants _and risking - what? Having to fertilize them? Most NPT enthusiasts would err on the side of "overstocking" plants.


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## Cawafuoshi (Mar 24, 2021)

I need to revisit Walstad's book for a second reading. She clearly concludes if I am not mistaken that the addition of fertilizers is not needed, that keeping fish is enough to provide for plants together with the soil, tap water, and light. Now that you coined the term "over stocking with plants", it is interesting for me to look at it as something detrimental to plant growth in such a system. "Heavily planted" versus "over stocked with plants".


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## Hank Junior (Oct 1, 2020)

Just wanted to say guys that though I regularly use liquid fertilisers I do not recommend them. Liquid fertilisers have many disadvantages and I battle green water alot. The only reason I use them is to keep my plants alive long enough until I work out how to do away with them completely. The Walstad/soil tank is far superior imo and is my main purpose. Right now the only great advantage I can see with liquid fertilisers is that they do create green-water which can help create an ecosystem of micro-fauna which live off this green water in a tank without fish.

I think the great strength and purpose of a Walstad tank is that it concentrates the nutrients in the substrate rather than in the water column and allows the mulm to be recycled into nutrients for the plants. A rich-nutrient water column is a problem for the aquarist. Add light and warmth and you get a very happy algae population.I personally don't think that fish or fish food are required but of course for most people keeping fish is their main purpose. As Diana says there are many ways to supply nutrients to an aquarium - fish food is one of them.


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## ronnie (Feb 26, 2021)

The short answer in regards to replenishing soil and nutrients is the breakdown of food and fish waste by snails, shrimp, and bacteria. (Which the shrimp and snails produce their own waste as well which is helpful).

It is for this reason that you’re encouraged to feed your tank generously.

Of course there are other circumstances at times. Overall though, it’s about creating a balanced ecosystem which allows for the breakdown of organics that can be further broken down by bacteria in the soil (providing nutrients in the soil for the plants).

There’s also some great backlogged threads about stocking. One of which was around 400% stocked. Understand though that this took additional maintenance and constant monitoring. It is possible to overstock a bit, as long as the parameters are staying in check. This requires great plant growth, and sometimes the safety net of a filter.

Again the overall goal is to create a balanced, self sustaining (mostly) ecosystem. And that can vary based on each tank based on plants, light, livestock, inverts, etc.


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## jatcar95 (Oct 30, 2019)

I think the overstocking is not as important as the overfeeding. After all, the nutrients from the fish originally come from the fish food - the fish just break it down a bit more. 

In a Walstad, the fish food *is *the fertilizer, so there's no need for liquid fertilizer. Even in tanks with just shrimp/snails, adding fish food is necessary to keep the substrate fertile. After all, if you're trimming and removing plants from the system, you're also taking out nutrients. They need to be replenished somehow.

The only issue I've run into with this is my greedy betta, who just tries to eat everything I put in the tank. I'm a little worried about overfeeding him, so I keep an eye on that. I have also used some root tabs when I was having some iron deficiency. Amending your fertilizer regime with substances other than fish food is definitely not a bad thing, but achieving the right balance of feeding/plant growth means less maintenance for us


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## Cawafuoshi (Mar 24, 2021)

Well, if fish food is THE main source of fertilizer, one could increase amount of food rather than the amount of fish for a boost in nutrients. But that might lead to fish over eating or the snail population exploding. The water quality shouldn't suffer too much in a heavily planted and perhaps filtered tank.


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## johnwesley0 (Feb 23, 2021)

Cawafuoshi said:


> Well, if fish food is THE main source of fertilizer, one could increase amount of food rather than the amount of fish for a boost in nutrients. But that might lead to fish over eating or the snail population exploding. The water quality shouldn't suffer too much in a heavily planted and perhaps filtered tank.


I just learned something from the last few posts. It sure sounds as if when people say, "Add more fish food.", more often than not, they are talking about fish-less tanks.


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## zahtar (Sep 29, 2019)

If you decide to stock heavily, keep in mind that the nitrogen cycle is not the only thing influenced by fish population. You decrease the available space per fish which might also lead to aggression. So I think that slowly increasing fish population will make it easier to monitor their behavior and decide accordingly.


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## jatcar95 (Oct 30, 2019)

Cawafuoshi said:


> But that might lead to fish over eating or the snail population exploding.


The only downside to snail population growth would be if you don't like the look of them. Once you find the sweet spot for the amount of food, the population will stabilize. I've never had any problems with snails, they just help break down the food so it doesn't go rancid before being incorporated into the substrate. The overeating problem is real though! I've gotten around that lately with some sinking pellets that my betta (and other fish) don't like. So only the snails and shrimp end up going for it, but it still adds nutrients.


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## ronnie (Feb 26, 2021)

It’s about finding the “sweet spot.” The balance between it all. It can be done in different ways, which is part of the beauty of it IMO. Whether it’s through stocking, feeding habits, etc.

Whichever way you go with it, you still have to remember the overall balance of it all. Amount of light and plants, with consideration of your livestock (or even if you don’t have any...) you still need to feed the tank. There is bacteria in there to break it down. It takes a bit of trial and error. Making small adjustments at a time helps combat any crash as well.


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