Dissolved Oxygen and Water Temperature

[Sylvia Bernstein]

I'm trying to figure out a good rule of thumb for how hot a fish tank can get before you drive out so much oxygen that not even a tilapia will be ok (under 4 ppm).  I know there are other factors like altitude and salinity, but generally do any of you science gurus know what that point is?  I think 100F is pretty much on the edge, but is 95F even ok?  Thanks for your help on this!

[TCLynx]

Well I have a chart here that tells the saturation levels at different temperatures but that only tells you what the saturation point is and can't really take into account the BOD and circulation factors.

At 100.4 F the dissolved oxygen saturation point in fresh water is 6.6  So if your system was very well circulated with lots of extra aeration and not heavily stocked, water very clean, tilapia can survive 100 F water but I wouldn't be feeding them anything when the water gets over 95 F because anything organic going into the water is going to be adding to the BOD.  I'm fairly certain my catfish and tilapia have survived 95 F water, at least in my old waterfall tank set up the water being pumped from the monster bed tended to be very warm on a hot sunny summer day. 

 

Now As water temperatures get up over 90 F at least with the catfish, I reduce feed a bit and if things start getting really hot, I might recommend going constant flood, constant flow and adding more water since a greater water volume and the constant flow/circulation/filtration will help with aeration and getting as much organics out of the fish tank as possible as well as stabilizing the temperatures and keeping them a touch cooler.  The switch to constant flow/flood can work at either temperature extreme to help keep it warmer in winter and cooler in summer and if the switch back and forth is easy one might run constant/flow/flood during the hot days of summer and go back to flood and drain at night to help cool things off and the reverse during the cold spells in winter. 

Sorry, that was probably more than you were really asking for.....

[Kobus Jooste]

Sheesh.

 

OK.  You have to take temperature, pH, time of day, altitude, water agitation, chemical processes, stocking rate and type of fish into account before there will be an answer on "safe".  I have taken my Koi to 88 F in an unaerated system, but there were only 20 smallish ones in 450 gal.  I have 40 tilapia in half that volume but there are three cascades aerating.  100 F will still be feasible for something like a some catfish or tilapia, as they can deal with very low DO.  I think most else will be floating without huge amounts of supplementation.

 

If the system is not pressurized, and you just want to know how much oxygen will be in water at a set temperature and elevation, then your answer will lie in Henry's Law.  This gives you the amount of oxygen that will be dissolved into a body of water at set temperatures and the atmospheric pressure (expressed in millimeters mercury).  Compare your calculation with known DO tolerance levels of the fish and you will be able to make some kind of rough statement.  This chat panel does not allow the generation of formulas, but it should be on the web, together with the solubility coefficients at 1 atmosphere pressure (an open vessel like a fish tank - will not work for pressure vessel) and the gas pressure for oxygen at your chosen elevation.

[TCLynx]

Yea good point.  Florida doesn't get any higher than 400 and some feet above sea level at the highest so what high temperatures I can get away with down here is a very well circulated system with additional air stones and air pumps is going to be different than some one a mile high.

 

Of course some one in a dry climate at a higher altitude will also have some additional means of chilling the water just by aerating and splashing it at night or simply blowing a fan at the tank surface which doesn't work very well in humid Florida.

[Sylvia Bernstein]

I was looking at Henry's Law yesterday, and I'm sure that will give the correct, very specific answer.  The problem is all the variables you mentioned Kobus.  I'm working on this for my book, and the audience is the home aquaponic gardener.  They (including me) are generally need a basic rule of thumb instead of a complex formula that will require meters, etc.  Most people don't have a DO meter, for example, nor do they know the atmospheric pressure where they are...although I suspect that you might ;-).  They are at a variety of altitudes. I'm looking for more like what TC is saying..."don't get above 100F or no matter how much agitation you provide you won't be able to force in enough oxygen for even a tilapia or catfish to survive (i.e. oxygen being less than 4ppm)".  My question is is there some scientific basis for that number being 95 or 100 or 105, etc.?  Maybe there isn't because there are too many variables...

[TCLynx]

I'd say there is Some scientific basis for the number Sylvia, it is roughly based on Henry's law and the particular species of fish but the margin of safety to what ever number you choose is going to be extremely variable because of all those other variables.  I mean if some one goes and sets their timer to run less often and doesn't provide supplemental aeration, as the fish get big, they will probably run out of dissolved oxygen even if the water isn't especially warm.  Perhaps instead of giving a "max" arbitrary temperature, you might give a "watch and start taking remedial action" temperature for different types of fish.  Perhaps for tilapia that might be 90 F, catfish and bluegill at 86 F, trout at 70 F.  Now the watch and take remedial action might be to first reduce feeding then to add aeration and increase circulation/pumping cycles or even go to constant flood, finally if things get too bad doing water changes or icing or hooking up a chiller if temperatures continue to rise beyond the really safe limits.  These are all things I've read about Rupe doing to get trout further on through the summer but they are not always successful due to many variables in the real world.

 

The only step I've ever had to take concerning heat was adding aeration and reducing feeding.

Sylvia Bernstein said:

I was looking at Henry's Law yesterday, and I'm sure that will give the correct, very specific answer.  The problem is all the variables you mentioned Kobus.  I'm working on this for my book, and the audience is the home aquaponic gardener.  They (including me) are generally need a basic rule of thumb instead of a complex formula that will require meters, etc.  Most people don't have a DO meter, for example, nor do they know the atmospheric pressure where they are...although I suspect that you might ;-).  They are at a variety of altitudes. I'm looking for more like what TC is saying..."don't get above 100F or no matter how much agitation you provide you won't be able to force in enough oxygen for even a tilapia or catfish to survive (i.e. oxygen being less than 4ppm)".  My question is is there some scientific basis for that number being 95 or 100 or 105, etc.?  Maybe there isn't because there are too many variables...

[Kobus Jooste]

What scares me is that if you leave out a complex explanation and just give a value, you will get jumped by a scientist lecturing you on your simplistic train of thought, or some newby will mess up the system completely, happen not to pass 95 on the way to a crash and then blame you for making him / her believe that the system is fool proof up to said temp.  I think DO deserves a bit more info, just as people keep a keen eye on total nitrogen without even knowing that it is the uniodized ammonia that they should calculate.  Still, I think you may get to a "speed limit" type figure if you do a couple of calculations yourself (see how close you can come to between 2 and 4 mg/L) and then explain to people that the figure is general and that they can familiarize themselves with Henry's Law if and when they need to know.

 

You may need to throw some values around until you get consensus kind of like how you got to the rules of thumb. I'm willing to chip in if it will help

Sylvia Bernstein said:

I was looking at Henry's Law yesterday, and I'm sure that will give the correct, very specific answer.  The problem is all the variables you mentioned Kobus.  I'm working on this for my book, and the audience is the home aquaponic gardener.  They (including me) are generally need a basic rule of thumb instead of a complex formula that will require meters, etc.  Most people don't have a DO meter, for example, nor do they know the atmospheric pressure where they are...although I suspect that you might ;-).  They are at a variety of altitudes. I'm looking for more like what TC is saying..."don't get above 100F or no matter how much agitation you provide you won't be able to force in enough oxygen for even a tilapia or catfish to survive (i.e. oxygen being less than 4ppm)".  My question is is there some scientific basis for that number being 95 or 100 or 105, etc.?  Maybe there isn't because there are too many variables...