gas exchange and tank covers

[BSBB]

I did a quick search on MFK about gas exchange, but I was unable to find a thread that addressed my question directly.

My question is simple and one that I've been pondering for a long time: If you keep a tank cover (glass/acrylic) across the whole surface area of your tank, does that impede to any significant extent the air/water gas exchange? I know there are quite a few folks who keep 'open-top' tanks (e.g., for plant growth), and I would appreciate thoughts on this question from you more experienced folks.

Thanks.

 

[Aquafreak]

I prefer to not cover my tanks completely for this very reason. I also run powerheads with venturi systems to aid in gas exchange. My opinion is based on some research done with koi tanks with and without covers. The tanks with 90% cover had a higher dissolved oxygen level versus being 100% covered. This information came from the owner of a koi/goldfish/pondfish breeder I purchased stock from when I ran a local nursery.

 

[nfored]

My 55 Gallon FOWLR is 99% covered. My 220G FW is also covered 98%, the fowlr has a protein skimmer that is infusing air into the tank so that takes car of this tanks exchange. The 220 gets its exchange through the sump. I mean think about it all the water in your tank comes into the sump so you can have you exchange in the sump or other filter devices if you have enough turn over through them. Just ask anyone who has an Aro or an EEL their tanks are as close to 100% covered as possible

 

[trizzot]

Normal breathing air contains 21-22% oxygen. Oxygen deficient is below 19.5%. As long as your water is exposed to air in any way, your fish will be fine. Many aquariums are completely closed on top. Im not losing another arowana because I gave him a jumping target. any time you expose your aquarium water to air, gas is exchanging. Even if your lids are closed, They are not sealed. You have nothing to worry about. Think about all the rooms you go into that have closed windows and doors.

 

[Aquafreak]

Well my previous comment was not meant to support an uncovered aquarium - because when I did have an arowana I had my tank buttoned up tighter than a.... you know what I mean. My point was that there has to be some type of water agitation via sump, hob, powerhead etc. Canister filters don't pull air into their systems and unless your output is agitating your surface water, you may not have ideal gas exchange. Another issue is that some aquarium treatment products can create a film on the surface of the water which will decrease gas exchange. Additional 'airation' can help to reduce this risk. Again I'm not advocating reduced tank cover for everyone because it's obviously not an option for everyone.

 

[BSBB]

Thanks for the input folks. I will be converting from my HOB filter to a canister filter (as soon as it is delivered - most likely today or tomorrow). Part of my (mostly) glass tank cover consists of a ~2.5" plastic/vinyl strip along the back meant to be cut to fit HOBs, heater cords, etc. When I convert to the canister, I'll have about a 2.5" X 7" opening where the HOB hung - that should be plenty of opening for sufficient gas exchange. I may even perforate the plastic stip a little by drilling a few small 1/8" holes along its length to help with gas exchange. It may not be necessary, but it couldn't hurt either. I do worry about fish jumping out of the tank, as ocassionally I see my Cory Cats quickly darting from the bottom of the tank to the top and back to the bottom. Crazy little buggers!

 

[Oughtsix]

I like my tanks completely covered. My wet/dry sumps do an excellent job at water oxygenation. This is the main reason I switched from canisters to wet/dry sumps... oxygenation! Canisters pretty much don't add any oxygen to the water. Your HOB will add a lot more oxygen to the water than a canister.

P.S. I run a canister along with my sumps.

 

[nfored]

I can say that I run my 220 with almost zero surface agitation and have never had a problem. I have only had a problem once in my fishkeeping life and that was a 55 gallon with only a canister filter, the return got bumped and pointed to the bottom so there was nothing to break the surface tension, lost 1 fish.