More oxygen needed?

[scottward]

I have learnt that fish heading to the surface when using CO2 enrichment does not necessarily mean "I have enough CO2".

Plenty of beginners with CO2 enrichment often comment something like "it can't be that I don't have enough CO2 because if I turn it up any more the fish head for the surface".

More likely it is because the dissolved oxgen levels in the tank are not high enough (beginners also of the mindset that the surface of the water should be absolutely still).

Ok, so I have a good surface ripple going.

But after a few hours into the CO2 enrichment period, fish start coming to the top, starting with the tiger barbs (high O2 requiring species).

By the next morning the fish are all swimming around normally again (i.e. the problem goes away temporarily; thereby ruling out anything else that might be in the water that would cause them distress).

Too much CO2 really just means that the fishes ability to capture oxygen in their blood is horribly impared as opposed to the CO2 directly harming the fish?

So it makes sense that getting the maximum amount of O2 in the water would be the aquarist's perogative, and then gradually add the well distrubted CO2 until the plants are growing well.

My tank is moderately stocked and I have 2 canister filters, so I guess the BOD (Biological/Biochemical Oxygen Demand) is reasonably high...

My indicator plant (H.Polysperma) doesn't do so well if I turn the CO2 down, but I don't seem to have any other means of adding more O2 to compensate (I already have a good surface ripple as mentioned earlier).

I guess this is where a sump and it's open nature (as opposed to closed canisters) gives you much more improvement?

Sump users that also have a DO meter, would I be correct in assuming that your DO levels are always very good? Therefore this perogative is met; you can then add as much CO2 as you like? Can you still overdo CO2 using a sump?

Scott.

P.S. I know water temperature plays a huge part too; here is Australia it is warming up now, my tank getting above 30 degrees celcius some days. I wonder if my tank simply cannot hold any more O2, perhaps it's saturated with O2 but that's still not enough? Are there any inexpensive ways to test O2 levels?

 

[dutchy]

I have used a Tetra O2 test kit in the past. Probably not very accurate, but if it says that the tank has 8 ppm of O2, I can reasonably assume it's not 3....

It's the same with almost every hobby measuring device, like pH controllers, drop checkers, etc. At the best it gives us a good indication. Still usable if we eyeball plants and livestock at the same time. Sometimes the latter works even better.

 

[Florin Ilia]

Sump users that also have a DO meter, would I be correct in assuming that your DO levels are always very good?

Here's what a sump user with a DO meter had to say:

7-8ppm without any photosynthesis, depends on temp.......this is at 100% saturation for those temps.
With lights and CO2, about 110 to 140% even.

I can't answer the other questions though - I'm not a sump user myself (yet!).

Florin

 

[scottward]

Thanks for the replies guys.

@Dutchy - you're absolutely right I think, like any test kit it might give you a ball park but that's about it. I wonder if ballpark would be good enough for me to know whether I do indeed have lower than optimal DO or whether I have 100% saturation and I don't have any choice other than reducing CO2.

@Florin - thanks for the detail from the earlier post. I've probably read it once before but forgotten it.

BTW - how can saturation be higher than 100% ??? Once the water is saturated with O2, any more added will just immediately bubble out of solution and the DO reading should never therefore be higher than 100% ? Obviously I am wrong, Tom is going to be right here, but I'm confussed about the 110 to 140% part!

Also -

I know that different fish have different DO requirements, but I wonder how fine the line is between enough/distress?

I'm pretty sure that I read somewhere else that fish can be suffering from (early/minor) respitory distress without showing any obvious signs like coming to the top and gasping. If the DO drops even more (or CO2 gets higher or something else that inhibits O2 uptake I suppose) then they'll start showing the more obvious signs.

So would a fish that likes DO at 6 mg/L be ok at 5.5 mg/L? Would they be ok at 5.0 mg/L? Or would they start showing distress at 5.95 mg/L? If the 'spectrum' is broad enough then I suppose a ball park test kit would be ok, but if it's a fine line then ball park won't be good enough I suppose.

 

[ShadowMac]

I think fish can adjust to varying DO levels within reason. Think of it like a person who lives in places of high altitude. The air is "thinner" and less O2 available per breath. People who live here do not notice, professional athletes often train in these types of conditions to improve cardiovascular efficiency. If someone who lives at sea level travels here and goes for a jog they would definitely notice the difference. I think with fish it is the same way. They can biologically adjust within a given range over a period of time. Too low right away and they may not be able to cope. CO2 inhibits the efficiency of hemoglobin binding O2, the greater the concentration of CO2 the more difficult for it to release CO2 from tissues and bind O2 from the water.

I have seen stress and illness cause a fish to decompensate. I recently had a blue ram that was at the bottom of the totem pole, others were picking on him and he didn't have a lot of places to hide from the others. He showed signs of CO2 or O2 stress by going to the surface, however, others of the same species that had been in the tank an equal amount of time showed no signs of O2 stress. He later died while I was on a work trip. Still no signs of stress in any other fish.

 

[Biollante]

Your Friendly Neighborhood Evil Plant Monster PITA/Old Gasbag

Hi Scott,

Saturation, or 100% dissolved oxygen (DO) at 30C is about 7.5-ppm (pure water at sea level, 30C, on a standard day is 7.56-ppm) if you have 8.25-ppm DO at 30C you are at 110%, should you have 10.5-ppm you are at 140% saturation, often referred to as “supersaturated.”


I have outdoor tanks and deal with high temperatures, I generally think it is better to increase aeration then worry about maximizing CO[SUB]2[/SUB]. At minimum, I would add vigorous aeration overnight.


With healthy critters in a stable system, the majority of our critters are okay down in the 2-4-ppm DO range. Many of us consider something like 4.9-ppm DO to be our redline number.


The first thing to consider is whether a critter (or plant) can tolerate the temperatures; I move plants and critters around based on the seasons. Some critters (and plants) simply cannot tolerate higher temperatures, no matter how much DO (or CO[SUB]2[/SUB]) is available.


The compounding problem is that the efficiency of our filters also decreases with decreased DO and increased temperatures. Each change of the season I track distinct changes in the system, literally new little bugs move in. I track dissolved organic carbon and find a definite spike and the associated algae, oddly enough I think them a help in the transition.


One trick that may help is dosing Hydrogen peroxide, H[SUB]2[/SUB]O[SUB]2[/SUB], two or three times a day. The Hydrogen peroxide certainly increases DO and gives a boost to the little bugs in your canister filters.


20-ml of 3% H[SUB]2[/SUB]O[SUB]2[/SUB] would be a safe minimum dose twice a day, the amount of organic compounds and the plants and critters make a difference but I dose at least twice that concentration three or four times a day.


Biollante

 

[1077]

If it were but one plant,(H.Polysperma) that did not appreciate lower CO2 ,and fishes and other plant's were alright,, I might consider a different species of plant that was more tolerant.
Believe Tom and other's ,suggest sealing sumps in some fashion, to prevent off gassing considerable CO2 if this is being injected but I may be mistaken.
With higher temps and less oxygen, I too would aerate of an evening when plant's are using up oxygen.
Just my two cent's.