i think i poisened my fish? help?

stanw

so this am all my fish were happy.. fed them a clam and some formula 1.
that was at about 8am at 3pm all the fish who like clam are on their sides in the tank barely still breathing. but the others seem ok? thought about doing water change but did that 2 weeks agos and have done nothing different.. it is 135 gallon tank. currently 4 out of 8 fish seem really bad...
i have a uv filter, bio balls, and a protien skimmer.. this is a fish only tank

gymnothorax

what fish do you have, which ones are acting strangley, what are your water parameters?