I don't know how good your SPCA is over there, but maybe a quiet word in your local branches ear about the fishes conditions and be able to quote each fishes tank size requirements, water temp and ph as each species will have different requirements. As well as stress to them the cruelty due to stress to the fish by having them in crammed conditions with the wrong types of tank mates. It might not stop the shop from selling the fish but if you make enough of a stink with your local SPCA they might be able to enforce better conditons for the fish kept in the store. I am pretty sure that one of the SPCA and RSPCA's creeds is "ALL CREATURES GREAT AND SMALL", well fish fall in to that somehwere so there has to be some types of leglislation regarding their store conditions.
Newspapers are always good to vent some steam, my local paper has an SMS page where people state all their gripes etc. Failing that talk to a reporter, most businesses usually see any publicity as good except when it drives customers from their stores. And a news paper wanting some news to fill it's pages might give your story some space.
In Australia we have two current affairs shows that are always looking for stories, your bound to have similar type shows over there and they might like to stick a thorn in a big corporations foot, our current affair shows seem to take great delight in messing with the big boys. In fact any time these current affair shows get a court ordered injunction they take great delight and make a story about that, wearing the injunction as some form of badge of honor.