I find that in small tanks the best way to inject DIY CO2 is to use the
upturned bell method rather than an airstone, powerhead or other method. This
way, once you figure out the right size of bell to get, say 25ppm of CO2, your
levels will not go much higher than this, even at the start of a new yeast
bottle. I do this on a ten gallon, and never see the level above 25ppm. At
the start of a new bottle, the excess CO2 just bubbles out of the cup and
doesn't have time to dissolve into the water (these bubbles tend to be big,
like an inch in diameter).

In article , "Tasslehoff"
wrote:
I see, I'm no expert on this but I'm fairly sure it's not a good idea
injecting Co2 into a tank this size. The PH swings alone will be seriously
stressing your fish and will be much more noticeable esp at the start and
end of the life of the mixture. 50-60 ppm is probably screwing up your
fishes ability to breathe ie perm damage to their gills. If you're really
determined to use CO2 try and aim for 1 bubble about every 10-14 seconds. ie
kill the yeast, maybe 1/10th teaspoon?



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"Insert witty comment here."
-John