Backyard Aquaponics
A place to discuss aquaponics

I try not to get involved in discussions of airstones because I believe they only have limited use ie transportation and emergencies and to a degree airlifts in certain applications.
DO is used by fish and CO2 is excreted, in a few circumstances Nitrogen is introduced. At normal atmospheric pressure (if there is such a thing given temp etc) there will be a balance between the gasses dissolved in the water and the air. Maintaining the balance involves gassing off ie exposing the water balance to the air balance. ie an imbalance in the water will correct itself at a water/air interface.
Sticking an airstone in a FT is about the least efficient way you can gass off.
That's it I am pissed and I have to cook again today and this is only IMHO, and have a nice holiday.

Yes you are correct. Strangely enough with 10% extra pressure at 1m of depth you can get 10% supersaturation and at 2m depth 20% supersaturation.



While you are correct that more air flow will allow the possibility for more gas transfer so will more pressure. I find it odd that you not understand this given that you seem to know what "the bends" are and I would have presumed how they are caused.



This unlikely scenario would only be possible if the fish were take the air directly into their bodies. Since most fish don't breathe air this is not a problem. If fish stay at the bottom for an extended period of time then the nitrogen in their blood will reach equilibrium with the level of nitrogen in the water. If we assume that the water at the bottom of the tank is 100% saturated (at 1m depth or 110% saturated at the surface or 10% super saturated at the surface) with nitrogen (possible but unlikely due to mixing) if that fish goes to the surface of the tank the nitrogen in its blood will then be at 110% saturated or 10% super saturated.

As you say fish are well adapted to dealing with this challenge and can excrete nirtogen from their blood much faster than we can. Plus the bubbles take time to form in the blood so fish can go to the surface and then return to depth without problems.

Gas bubble diving disease in fish is a known risk in AQ and as such some time is spent ensuring that nitrogen supersaturation is minimised. Since the risks and causes are known systems are designed so that air is never injected below water at a depth greater than the depth of the FT.

if fish begin to feel the effects of nitrogen bubbles forming in their blood they simply swim deeper the problems occur if they can't swim deep enough. At first this is just irritating and uncomfortable but even mild cases can cause damage to the gill filaments of fish. This won't kill them but it decreases food conversion rates, health in general and increases the chances of gill infections. If the problem persists or is more severe then it can kill the fish outright.

This is rare because aquaculture professionals know not to inject air under pressure. It still occurs though and can result in serious mortalities. Most often this occurs when a circulation pump develops an air leak on the suction side and introduces air into the main distribution line. Not good for the pump (due to cavitation) and certainly not good for the fish. There are other causes like connecting an air line to an oxygen line or other problems caused by failing or poorly maintained equipment.

While AQ designers know what they are doing and so know not to add air injection at depths greater than the FT, amateurs on this forum and others do not have the benefit of their education and industry experience. The injection of air at increased depths has been suggested by a number of posters on a number of different threads. Therefore it is important to not only tell the forum members, particularly the new members, not to do this but why not to as well.



You can do the calculations but you would need to know all the parameters. While under some conditions 2/3rds of the O2 may have dissolved into the water under others it could be almost none.

I am not sure the air lift diagram holds up with the bends problem. The air lift outlet is not at FT bottom depth it is at the surface of the FT. Extra N will come out of the water before the water gets back to the bottom of the FT, is what I'm thinking. If the air were introduced below the FT bottom then the outlet were at the FT bottom then I think the bends problem could be in play?

I've have been a sport diver for some time and have a good understanding what "the bends" are, although I never experienced it myself. There is a clear and very informative wiki about it, so I leave it up to them to explain the ins and outs in detail. See this link http://en.wikipedia.org/wiki/Decompression_sickness

More air pressure at, lets say at 1 meter, will do nothing for the DO. In order to get more DO with higher pressure, you need to inject at greater depth. Of a given volume of air, more of the O2 (and also the other gasses contained in the air mix) will become dissolved into the water when it is injected with higher pressure at a greater depth. So if you inject air at 10 meter deep, you need to overcome that pressure with a bit more than 2 bar so the air will flow.

If you would transfer that water in a sealed container to the bottom of your FT at 1 meter deep, and then open that container the dissolved gasses will immediately start to come out of solution in order to reach equilibrium with the now lower pressure at 1 meter (2 bar minus 1.1 bar is a pressure difference of 0.9 bar). If a fish would suck on the outlet of that container, it would not get the bends but possibly blow himself up like a balloon.

If that same water sample would be transferred from 10 to 1 meter by a pump and pipeline, the sample will release the dissolved gasses gradually while the surrounding pressure decreases during the ascent to 1 meter. When the sample reaches the FT bottom at 1 meter it is already equalized with its surrounding water. It cannot harm the fish that are swimming there.

Nitrogen is for humans the most likely problem with decompression, because nitrogen is much slower to come out of solution than the other gasses contained in air. That's why some divers can even get "the bends" 24 to 48 hours after they ascended from the deep without sufficient decompression stops.

I wonder, and actually doubt it strongly, wether nitrogen would be an issue for fish at all.

What can happen though with injecting at high pressure into deep water, is an over saturation of DO in the water. But this is because that practice is done with injecting pure O2. And to much DO in the water is not good for the fish! They die of it. It's in fact the same effect as taking the fish out of the water and leave it in the air. It will practically die because of an overdose of O2. Which could give the same symptoms as the bends, but is still an other kind of problem than decompressed nitrogen in the body of mammals.

I agree with you, that if you don't know what your doing with AQ or AP, don't! And since we are dealing with living beings (fish and plants) double caution should be taken.

I find this logic very odd. You seem to be saying that injecting air at a depth of 10m will increase the levels of dissolved gases in the water but injecting at a depth of 1m will do nothing? Will 2m also do nothing? What about 9m? What about 5m? Surely it is a gradient with the waters capacity to dissolve gasses increasing with depth or pressure?

What you are saying goes against masses of industry knowledge, literature and experience accross a wide range of applications. For example there are other forms of gas exchange devices called packed column filters which can be used as bio filters, gas exchange devices or both. These devices are some sort of vessel, usually a large diameter pipe or narrow tank, where water enters at the top and trickles down through a series of plates or media. To increase the ability of these devices they are pressurised by having an air pump pump air into them and the exit of the air being restricted so that a constant pressure is maintained in the packed column. Increasing the air pressure in the packed column to 0.1bar gauge increases the ability of the water to absorb O2 by about 10% from ~9.1mg/L to 10mg/L (at 20c and sea level). These devices come with strict warnings to not increase air pressure above 0.1bar because of the risk of fish diving disease (nitrogen supersaturation).



It is precisely because the dissolved gasses come out of solution gradually that problems occur. When a pump is pumping what is the velocity of water moving in the pump? In a system designed to reduce friction losses the flow is going to be relatively slow ~1m/s. Therefore water from 10m depth only has 10s for the partial gas pressures to equalise. Not enough time.



Which again reinforces the danger. Fish constantly exposed to nitrogen supersaturated water at low levels will be chronically affected. This may only be an added stress but can easily result in actual harm to the fish. The most minor and common form that this takes is the tiny fragile gill filaments being damaged. Tiny bubbles form in the blood and become caught in capillaries of the gills. Once trapped there a bubble will increase in size and continue to absorb more gas from the blood and other bubbles. Well before this the fish will have gone as deep as possible to minimise the effect but if the depth of the tank is insufficient eventually the bubble will burst the gill filament. A few of these is no big deal but the more it occurs the more the fish will be prone to infections and the less efficient its gills will operate further compounding the problem.



From "Physiology of Fish in Intensive Aquaculture Systems" by Gary Wedemeyer




Note the eventually and note the 110% (ie from injection of air at 1m) saturation.


What can happen though with injecting at high pressure into deep water, is an over saturation of DO in the water. But this is because that practice is done with injecting pure O2. And to much DO in the water is not good for the fish! They die of it. It's in fact the same effect as taking the fish out of the water and leave it in the air. It will practically die because of an overdose of O2. Which could give the same symptoms as the bends, but is still an other kind of problem than decompressed nitrogen in the body of mammals.

I agree with you, that if you don't know what your doing with AQ or AP, don't! And since we are dealing with living beings (fish and plants) double caution should be taken.[/quote]

I'm curious about how gas releasing from water will affect the fish. If water is coming up from 10m with DO & DN etc. wouldn't the gas be forming bubbles and therefore have little effect on the fish?

I've read innumerable times that if you are deep and have to come to the surface and don't have the portable supply to allow staging areas, to avoid the bends you should rise at the speed of the bubbles, mouth open to allow the gas to escape from your system. How true this is I don't know but I've seen it in everything from Sea Hunt to Navy and Dive films.

Now fish must have mechanisms to cope - AFAIK the problems for fish come when we haul them to the surface at the end of a line or in a net - left to themselves they happily transfer from far greater depths than any of our tanks.

Given the pressures involved in normal AP and AQ, is there any real chance that injecting air at 10m down might harm fish swimming in 1m of water? (serious question - I'm interested in the physics and technical side but most people aren't - they just want to know if what is being talked about here might harm their fish)

Seems I've started something well over my head here!!

I've just gone with the air stones hanging halfway down the outside of my SLO and there is heaps of turbulence/ bubbles on the surface now, fish & plants are healthy so I'm happy


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Hi Stuart, have to get on with my project so will try to answer tonight (local time). Although this pleasant discussion is not really answering the OP's question, it is interesting enough.

Cargs, I think if you put your air stones at the bottom and your pump is doing about 30 liters/m you should be fine. Can you post the specs of your pump?

It takes time for the gas dissolved in the water to come out of solution.



I don't know much of anything about diving but I believe that you have to come up much more slowly to give your body time to purge the excess nitrogen from your blood. You want to come up a bit at a time so that the nitrogen is being exuded from your blood in your lungs and then expelled rather than forming bubbles in your blood.

If you take a bottle of soft drink as an example if you release the pressure quickly the bubbles form in liquid and you get the bends. If you release the pressure just a tiny bit then you won't see any bubbles (if the tiny bit is tiny enough) but the patial pressure of the gas dissolved will be less than the pressure of the free gas at the top of the bottle. This will mean that the gas dissolved will come out of solution only at the boundary of the soft drink and air. Once equilibrium is reached you can release a tiny bit more pressure. How long would this take so that you didn't see any bubbles? A long time. Any shorter and you get bubbles which for the sake of the example means bubbles in your or the fishes blood.


Yes they do. They dive to the bottom of the tank, hence the name it sometimes given "diving disease" because the fish dive to the bottom.



Given that the recommendation is to never inject air below a depth of 1m then I would say that 10m is likely to kill fish outright. The most likely scenario where fish are killed from nitrogen supersaturation is where a pump is ingesting air from a leak on the suction side or from being sucked in the end of the suction pipe due to a lack of sufficient submergence of the inlet. This results in air being subjected to maximum pressure just after the pump. Say a system has a total dynamic head of 6m (not unusual for a RAS) this is the pressure at which air from the suction side is injected into the flow. If such a relatively small amount of air at a relatively small pressure (6m vs 10m) can kill fish then I can easily imagine injecting relatively large amounts of air below 1m will cause problems if the FT is only 1m deep.

In answer to Dave: yes having the outlet at the bottom would be worse and having the outlet at the top is better but still not good enough. What you want to have happen is the reverse of oxygenation. If having the outlet at the top is not sufficient to add enough oxygen to the water (if being pumped by a mechanical pump rather than airlift) it is unlikely it is enough to allow excess N2 to be removed. Now the incoming water will mix with what is in the tank so even if the incoming water is at maximum saturation on entering the tank the concentration will be diluted on mixing with the tank water. So you might say that would allow us to place the injection point deeper especially if we are trying to pump with airlifts but AQ professionals know about mixing and the rule of thumb of not to inject air below 1m takes this and other factors into account.

For example smaller fish are more susceptible to this problem than larger fish. Hatchery, fingerling production operations are recommended to not have N2 saturation over 102-103%.

Do you know if anyone has given an explanation as to why something like this may happen in aquaculture and not say at Niagra falls, where you would have "air injection" much lower than 10m and healthy fish populations?

It can and does.

Remember the water is fairly deep there so fish have some scope to dive and also many fish can detect supersaturated water and in the wild will avoid it.

Personally I feel that the fish at the bottom of Niagara Falls are at more risk of being hurt by a McBarfit slushie hitting them on the head than nitrogen supersaturation.

I have to agree with Stuart on the whole nitrogen supersaturation subject. These are known facts that have been studied and documented since before I was born. We actually had a DO sensor installed in our Speece cone to alert us if Oxygen levels dropped below 92%... Because if they did there was either a leak in the Venturi, pump, piping or problem with the O2 generator. It doesn't take long until you have a massive fish kill... Especially when running densities over 1lb/gal.

On the question of "is this something we actually need to worry about in AP or AQ", the answer is YES. Hobby systems aren't going to see those kinds of pressures, but when you get into large scale AP or Aq and you are trying to optimize space by increasing fish densities using O2 and a saturating device like a speece cone or other pressurized vessel, it is a real concern.

Ryan can you back me up on the risks of injecting air at depths in the case of a sump that is deeper than the FT or in an airlift or U-tube that is deeper than the bottom of the FT.