I have a 21 gallon freshwater tank with about 40 tetras in it and some shrimp. My only pencifish's left eye is bulging out for some reason...what can i do about it??
As for your fish sounds like popeye. Take a look at This and see if that looks like what he has. Popeye is a bacterial infection of the eye. So you will need to go to your LFS (local fish store or pet store) and look for medication that says it treats popeye or says it's a bacterial mediction.
pop eye is a symptom and not a disease in it's own right.
it is often caused by poor water quality.
check you core parameter (ammonia, NO2 and NO3)
and address the out lying cause.
if the core parameters are ok then it could be any number of bacteria that are causing it.
treatment would be with melafix for 7 days.
I have to suggest that 40 tetras in a 21g tank is a lot-- probably way too many. If your ammonia/nitrites/nitrates or high I'd bet the cause is overcrowding. You may wish to consider rehoming some of them or getting a bigger tank.