# Large water changes & decompression sickness



## Yo-han (Oct 15, 2010)

On a Dutch website I found an article about the danger of large water changes. I translated the section with Google (see below).

The first part is about O2 and CO2, the second about N2 gas in the water and in fish.
In short it tells that even a little oversaturation of N2 in the water can cause problems, and supersaturation (>105%) is able to kill fish. Basically the same thing happens when a diver gets up too fast -> N2 gas bubbles in the blood (decompression sickness).

When water comes from the tap at 10 degrees celcius (and under high pressure) it contains a lot of nitrogen gas. When it gets heated to 25 celcius and room pressure the amount of N2 gas is much higher than 100% saturation. When only doing a 30% water change you already have 118% saturation. Way too much and I do larger water changes quite often.

I can't find any English or other info about it nor have I ever experienced this myself. But I do have lots of air bubbles in my glass after a water change (which I always assumed was oversaturation of oxygen but might as well be partial N2 oversaturation). I'm curious what other people think about this...

_Google Translated version of a part of the article, the original article in Dutch can be found here:
http://www.vdheuvel.org/adriaan/gasseninwater.html
__

The control of breathing

For a long time we thought that the respiration of the fish is regulated by oxygen (O2) levels in his blood. Today, this is already out again, a fish also has CO2 receptors with which it can calculate the CO2 levels in the surrounding water. But still breathing for most of the oxygen in the blood is regulated. CO2 receptors are hereby seen more as an early warning system.

In nature, a high CO2 content often an announcement of an upcoming shortage of oxygen. As a fish can set early on lower oxygen levels.

When our people breathing is controlled by CO2, fish so mainly on O2.

Little oxygen in the water, the fish breathing rapidly, the heart rate goes down, as well as the level of activity.
A lot of oxygen into the water, the fish breathing slowly.

A larger amount of CO2 in the blood has a number of consequences:

Lower pH of the blood due to the extra CO2
The CO2 hinders the absorption of oxygen (Root effect)

Anything explanation according to what extremes, which usually works the most illustrative:

1.
Do you have many O2 and very low CO2 than fish breathes very slowly. CO2 diffuses then very easily from the fish (high concentration) in water (low concentration). The CO2 content drops in blood. Due to the buffer effect of the blood and slow breathing is of the effects of the low CO2 content (higher pH), still a little. But compensation is not quite possible. So pH of blood increases to normal.

The combination of a lot of oxygen and very little carbon dioxide is not as good as you might think. In our aquariums can avoid this situation, for example, in cases of biogenic decalcification in which the plants to get the last bit of carbon dioxide from the water.

Oxygen saturation values of about 200% can by the oxidative action of oxygen very large even cause damage to the gills within a time frame of 6 hours. A little oversaturation can not hurt to exaggerate.

2.
There is now very much very much O2 and CO2 (for example, a large planter with CO2 fertilization) Fish breathe slowly. The high CO2 content in the water will also internal CO2 levels are already relatively high. The pH in the gills is thus lower than you would expect. And in fish with a strong Root effect, there will due to the low pH even less oxygen may be included in spite of the high oxygen content in the water.
By the slow breathing caused by high oxygen can fish the internally produced CO2, another worse lost to the water so the blood pH drops even further and the fish is in trouble due to low blood pH.

This situation is also known as respiratory acidosis. The situation of a large reduction in the pH due to high blood CO2 levels is also known as Hypercapnia.

The kidneys try the low blood pH straighten again by bringing HCO3- in the blood. However, it takes a long time, the pH will be in order, the production is very limited. In continuous low pH values caused by continuous high CO2 levels can kidney damage occur.

3.
There is little O2 and CO2 in the water the fish breathes very fast. The CO2 level in the blood of the fish rises. This is partly offset by rapid breathing nations but blood pH will drop.

The heart rate is back to get more time to absorb oxygen from the gills. The fish unscrew its activity back to energy, thereby saving oxygen.

4.
Low O2 and low CO2 Fish breathes quickly. CO2 is very rapidly removed from the blood. The pH will increase significantly and the fish gets into trouble.

Especially with low oxygen levels and very little CO2 blood pH which can grow to very high pH values rise so the fish is in serious trouble. This situation is also known as respiratory alkalosis.

We see, then, on the basis of the examples that situations 1. and 3., but still be acceptable, and 2. 4. not. What now occurs in nature for ........?
Daytime >> much oxygen production >> >> so much CO2 uptake situation first.
Night >> much CO2 production >> >> so much CO2 uptake situation 3.

Anyway, it is obvious ..... much O2 and high CO2 is not all ..... this is immediately forced major drawback of low pH with CO2 because we create two situation.

It is best therefore, to follow the natural as much as possible in its fluctuations.
nitrogen
Nitrogen bubbles in a gill 11.6 kB

The worst thing we can get is an over-saturation of nitrogen (N2) into the water.
Fish can say, get like people, the divers disease or bends. In English, the disease is also known as Gas Bubble Disease. Google just once on.

At a certain pressure, there is also a certain amount of dissolved nitrogen in the water. We now reduce the pressure very quickly then there is question of a temporary over-saturation of nitrogen in the water. For example, when a thunderstorm will flow through the low atmospheric pressure of nitrogen from the water to the atmosphere. The water is then saturated with nitrogen.

In addition to the pressure, the temperature also plays a role. Indeed, in cold water can dissolve more nitrogen than in warm water. In cold tap water can thus by the higher pressure and low temperature with respect to our aquarium water is much more nitrogen.

The tolerance of fish for a nitrogen supersaturation is very limited.
As low as an oversaturation of 102-103% can cause problems. That's surprisingly low!
At a supersaturation of 105-110% is already a nice chance of fish kills. Especially fry is very sensitive to a supersaturation.

With a small over-saturation is a fish still able to swim to deeper water layers. Since the water pressure is larger. For every 1% supersaturation must swim a fish 10 cm deeper to compensate for the effects of supersaturation.

When a supersaturation of 5% should be a fish so all swim at 50 cm depth. This is difficult in a cup of 40 cm.
Protruding eye by excess nitrogen 7.6 kB
Nitrogen saturation, when?

The greatest risk of saturation, we at:

thunderstorm
If the pressure from 1015 to 1005 mbar drops then it goes N2 content (according to the calculator on this page) of 14.1 mg / L to 13.96 mg / l. That's 0.14 mg / l less. A supersaturation of 0.14 / 14.1 x 100 = 1%, that effect is not too bad. Greater the difference if we get a thunderstorm during a good weather day (high pressure area).
water Refresh
This is for us aquarists the biggest problem.
The water pressure in our pipeline is approximately 2-2.5 bar. (2000-2500 mbar)
If we fill our tank with tap water, we see it all off small bubbles.
That's no carbon dioxide, usually oxygen. But nitrogen.
Yes, by changing with tap water so we create a supersaturation

example:
Tap water 2500 mbar of 10 degrees at 50% saturation (Tap water is seldom 100% saturated) gives 22.8 mg / l
Aquarium Water 1013 mbar of 25 degrees at 100% saturation gives 14.07 mg / l
If we change 30% then we 0,3x22,8 0,7x14,07 + = 16.7 mg / l
That's an oversaturation of 100x (16.7 / 14.07) = 18.6%
That's clearly too much! So before you tap as topples into the bin. First, a couple of hours is well ventilated so that it comes into equilibrium with the atmosphere. Marine aquarists aerate not for nothing as long refresh their water!

A clear example of Gas Bubble Disease








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## Michael (Jul 20, 2010)

Interesting! I too have often noticed the bubbles when doing a water change with cool, fresh but dechlorinated tap water. I suspected that supersaturation was the cause, but thought it would be CO2 rather than nitrogen. And I've always suspected that it was dangerous to fish, so long ago started aerating tap water for 24 hours before using it for large water changes.


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## Yo-han (Oct 15, 2010)

Well, I'm quite sure it isn't CO2 because the saturation point of CO2 is around 1500 mg/L. If it was, your fish were long dead. Perhaps I need to collect the bubbles and test whether they're O2 by holding a flame or something. Don't know if that would work.


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## illustrator (Jul 18, 2010)

In some areas, where there is a very high pressure on the water in the pipes, the water becomes white when you poor it in a glass. the white colour is caused by minute bubbles and disappears in a few minutes. I think that this water is oversaturated with gasses (actually just "air") and may be dangerous for fish. I think that airating for 24 hours is OK, but that half an hour would also do.


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## BruceF (Aug 5, 2011)

Some of that is temperature.


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## illustrator (Jul 18, 2010)

Yes, cold water can contain more dissolved gass than warm water, so if your tapwater comes out very cold and you mix it with hot water, you can get the same effect. And the same remedy to solve it. 

Still, if you let the water plunge in a bucket from some height (50 cm, from the shower head for instance) you also lose much of the excess gass already.


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## BruceF (Aug 5, 2011)

I know I read last year Tropica was dumping carbonated water into their show tanks. I assumed they did that before they put the fish in.


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## TropTrea (Jan 10, 2014)

It is an interesting article. However I few points that need to be considered.

1. Do you pump water directly from your tap into the tank? Or do you run it through a RO or even DI system into a holding tank that has a heater in it as well an air stone. If you do it as I do then a majority of that article does not apply. 

2. Even using the system I use I only go over 20% on a water change in emergency situations. As I see it the Kh,Gh and pH etc. may not be perfectly matched so your changing the water chemistry with any water change. Even if the water change improves the conditions for the fish you are subjecting them to shock as they have to readjust to new water conditions. With small water changes the shock effect in minimized.


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## Yo-han (Oct 15, 2010)

Yes I pump tap straight into my tank. KH is almost 100% stable through the year here. No chlorine etc. So I only adjust the temp to the temp of the tank (0-2 degree colder). 50% every week. I do get that if you aerate it or RO etc. it isn't a problem. But what about my method? And that of many I guess...


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## illustrator (Jul 18, 2010)

I change about 30% at once, my tap water is local spring water, hard and in winter only a few degrees above zero (centigrade  ). There is no "water company", the water system is owned by the county. All the county did is add a filter and test water parameters. So there are no additions here either. I heat up the water with a household boiler and take mixed cold+warm water from the tap. I use this directly. 

I have a rainwater barrel and used this some time for a softwater setup, but the plants which I really wanted to grow didn't do well (presumably because I used beech leaves from the forest behind by house, which is on karst underground). So I am back to growing hardwater species.

I think that fish receive bigger changes in water chemistry during tropical rain-showers.


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## thunderjack14 (Nov 28, 2014)

I do a 50% water change every week with a python system i try to keep the same temperature as the tank water. I add prime to the water at the same time as i fill tank. Bin doing this for years now with no side effects of the fish. if anything the fish seem more active. But then again my water out of the tap is right where i need it to be as far as KH ,GH ,PH.


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## Yo-han (Oct 15, 2010)

But are they more active because they don't like it, because of the nitrogen gas or stress, or are they really happier? For example, when you move kuhlii fish, they fly all over the tank. Not because they are happy, but because of stress.


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## Zapins (Jul 28, 2004)

Yo-han, I had this exact problem a few years ago when I first set up my auto-water change system. Here is a link to the thread: http://www.aquaticplantcentral.com/forumapc/fish-planted-aquarium/65986-pop-eye-too-clean-water.html

I was changing 200% or more per day on my 90g tank with well water, but it took about 3 weeks for my angel fish to develop popeye (decompression sickness). I'm not sure if this is an average amount of time, or what the lower limit for seeing the issue is so I can't really generalize my experience to normal practices.



Zapins said:


> Haha, well I somehow managed to give my fish pop-eye from keeping my tank water TOO CLEAN!! I have had my auto-water change system running 24/7 for about 3 weeks now and I just noticed that my angel fish's eyes are popped out of his head. Wondering what the heck could cause this problem in such a clean tank (~200 gallons changed per day on my 90g) I found the following information online:
> 
> "Pop eye can also be caused by gas bubble disease as a result of oxygen super saturation (excess levels) of the water with the gas, nitrogen. Super saturation occurs whenever the pressure of a gas in the water is higher than the pressure of the same gas in the surrounding atmosphere, whereby the difference in gas pressures causes the gas to get pulled too quickly out of the fish's bloodstream, leaving behind gas bubbles. The other symptoms of this are the appearance of bubbles under the fish's skin. It's caused by excess oxygen in the water, particularly from filters that blow air directly from outside to inside the tank, and from pressurized tap water that did not get mixed. "
> 
> ...


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## 1077 (Nov 7, 2011)

Have raised Discus and other cichlid's from babies to adult size fairly quickly with three to four feeding's per day and three 70 to 80 % water changes per week.
To be sure,,without the three or four feeding's the water changes could have been less frequent ,but judging from growth of the fishes,weight increase weekly of fishes,continued spawning activity,I am doubtful that large water changes are the boogey man unless...they are far and few between.


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## dirtmonkey (Mar 12, 2007)

I'm calling this a myth. There can't be any risk of decompression sickness or 'bends,' because there is no decompression. That already happened the split second the water is out of the tap, before it even hits the container. The worst that can happen is the bubbles annoying the animals. Granted, that's enough for me to sit the water out or aerate it for a while, but that's the only reason.

On the temperature issue, same thing. It's already done by the time it hits the fish (if not, you have other problems).


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## Yo-han (Oct 15, 2010)

I found a book that mentions nitrogenembolism. They mention a little different numbers but still the fact (/myth) remains. They say O2 oversaturation isn't hurtful until 350% oversaturation and N2 supersaturation: 103-118% depending on the species. Their solution is running the water through a sponge or something similar and this takes away the problem. I doubt that would make much of a change, but OK. Easy solution...


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## Alasse (Jan 5, 2011)

I do 90% water change on my goldie tank weekly in warmer weather, they act no different before the change to after the change
In colder weather they get 30% WC three times a week so the temp shock isn't as high


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## Michael (Jul 20, 2010)

1077 and dirtmonkey, the danger is not from large water changes per se. The danger comes from large water changes using water that is supersaturated with gases. This only happens under specific conditions. As for the decompression occurring as soon as the water comes out of the tap, this is plainly not the case because you can seen the tiny bubbles coating the surface of objects in the tank and rising through the water to the surface.


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## dirtmonkey (Mar 12, 2007)

Yes... you can see the bubbles of escaping gases because the decompression has already occurred. If it hadn't, there wouldn't be any bubbles  

Once that is poured and mixed with tank water, the excess gases are in such small amounts beyond partial pressure equilibrium that they can only create some "false pearling," and after that there are still the bio-regulating membrane barriers to getting into any of the critters' bodies. Gas embolism will not happen, but irritation from the bubbles themselves might in a worst case.


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## dirtmonkey (Mar 12, 2007)

I should be clear here that I'm talking about normal tank conditions, not just being argumentative. Yes, supersaturation can happen and cause problems experimentally, but those experiments had to be run carefully to preserve the dissolved gases in the water. It's the actions of coming out of a tap into air (no longer under pressure), and further pouring and dilution that we normally do makes it not a practical problem.


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