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RE: air at night

I use an airstone at night in my CO2-injected tank. This adds O2, mostly 
via surface turbulence, and drives off much of the CO2. Here are the pros 
and cons of this approach as I see them:

PRO: Fish never suffer from oxygen deprivation. Fish seem healthier and 

CON: Expense (air pump and timer). Noise (air pump and diffuser). CO2 is 
driven off and wasted. The loss of CO2 causes a pH increase that may stress 
the fish.

I think this is a controversial practice; many people say that with healthy 
plants you never need to worry about oxygen. I've found this to be true, 
but with a major exception: if anything goes wrong with the plants, the 
fish can suffer due to lack of O2. A major water change, an algae or 
nutrient problem, a major pruning can all cause this to happen the next 
night. Also, if your aquarium has a high fish load or less than a "Dutch" 
amount of plants, it can happen regularly.

All I know is, the pH swing due to the loss of CO2 is minimal and gradual, 
and I've never had any fish stress or deaths due to pH swings. I *have* had 
fish stress and deaths due to O2 starvation, sometimes even when the plants 
were bubbling like
crazy the day before. I haven't lost a single fish in the 2-3 months I've 
been running the airstone at night.

If you're considering this, try watching your fish at 1:00 am for a few 
days. In my case, even when the plants were bubbling like airstones, the 
fish would all gather at the surface gasping for air a few hours after the 
lights went off. This usually didn't kill them, but it was stressful and I 
would occasionally lose a fish--and always a fast-moving fish that needed 
high oxygen levels.

I suspect this is mainly a problem that those of us with relatively high 
fish loads run into. I like to keep my one inch per gallon of fish, or a 
bit more, even in a planted tank. Amano's fish loads tend to be a bit high 
and some of his designs use a relatively small number of plants, so I'm not 
surprised that he does this too.

Injecting *pure* O2 is a novel idea; I imagine this could be done silently 
and, with a good diffuser, wouldn't drive off CO2. However, this would be 
expensive. More importantly, my aquaria are definitely NOT worth the risk 
of keeping a canister of pure O2 in the house...

michael moncur   mgm at starlingtech_com   http://www.starlingtech.com/
"Let's have some new cliches."                -- Samuel Goldwyn