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Co2 in covered tank

nigel mccarthy

Seedling
Joined
3 Jul 2016
Messages
4
Location
Basingstoke
Hi all, I've recently redone my 55 gallon tank, got the co2 dialled in and plants are growing well and looking healthy.

I have just got myself a sheet of acrylic custom cut to fit the top of my tank as I left it too long to top up the evaporated water last time it was set up and got massive calcium deposits all around the top which was a massive pain to get off.

My question is will the amount of co2 in the tank change massively now that it's covered and there is nowhere really for the co2 to escape when it reaches the surface? I don't want to come home from work tomorrow and find I've gased all my fish!!

Many thanks
 
CO₂ is heavier than air so any that escapes from the tank from the water surface will form a layer just above it where it can then recirculate back into the water if it is trapped there. If you were borderline CO₂ safety for fish before the lid I'm fairly certain you will be beyond it after (even if it's loose fitting). I'd dial the CO₂ down a bit or alternatively a better idea is to wait until you have a free day to monitor the behaviour of CO₂ injection after fitting the lid.
 
Cheers for the advice, I thought it would make a difference!! I've dialled it back a bit and will monitor over the next few days, luckily I get home from work an hour or two after lights/co2 come on so can make any further adjustments if needed.
 
Hi Nigel,

I suggest you also keep a close watch on dissolved oxygen in the tank water. As gas exchange will be limited by a close-fitting cover, atmospheric oxygen will not freely enter the water. During the day, plants take in CO2 and give out oxygen. At night, the process reverses - so oxygen level in the water will reduce. Maintain dissolved oxygen at 5 - 7 ppm. Note that tropical freshwater aquarium dissolved oxygen saturates at 7.5 to 8.4 ppm.

You may find that your acrylic cover bows in the presence of water vapour and warmth - as I found out. I'm not sure if polycarbonate would have been a better choice but I'll probably try that next time. If your acrylic cover does bow, it may create leaks around the edges. If I understand you rightly, that's what you're trying to avoid. But it should help with gas exchange!

JPC
 
Cheers Jay,

The lid isn't close fitting at all, it just sits on a lip I've got around the top of the tank from the original lid I got rid of when I upgraded my lights, there are also cut outs in the back corners for the intake and output from the filter so there is a small area for gas to escape, the acrylic sheet is mainly to reduce evaporation which so far has been almost none where normally I'd be topping up a full 10 litre bucket twice a week, so far plants and fish are looking good and once I sort the diatom outbreak I've got I'll be happy!
 
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