My best suggestion is to use the complaint form on the actual website and lay out your complaints there, saying that you won't shop at that store again until the conditions are improved. I'm not a confrontational person, so I can never really go up and complain to a manager, and I've worked in retail too long to ever complain to a standard employee because I know that there are some things they would
like to do, but are not allowed to. Also, if you complain over-much, that's the point when people working in retail start nodding and apologizing--and once you leave, they go find whatever other worker isn't doing anything and start talking. The conversation usually begins with "You wouldn't believe this customer I just had..." Even managers will do this sometimes, though they talk about it after the store's closed for the night.
Now, I know that there's a big, BIG difference between the guy who complained to me for ten minutes that there was an erotica section in the chain bookstore and someone genuinely protesting the treatment of live animals. But most employees won't care. Even those that work at a pet store might not like fish, so they might look at the fish as not being any different from books. Also, threatening someone with reporting it to corporate might not be the best idea, because (a) if you get too worked up, they will remember you when a complaint comes in, and (b) threats to report things don't have a lot of weight. I had one woman tell me that she was going to make sure I got fired when she misunderstood something I said to another employee, and I never even got a cursory "watch what you say to customers" note from my boss. No write-up, no nothing. I'd just report the fish treatment, and if it doesn't get better or the problems start occurring again, report again and get some of your friends to do the same. Not only is the volume of complaints important, but reporting something online means that your age doesn't matter--which it does in-store, unless you look over 25 or so (i.e. out of college).
I'm sorry, 1fish2fish, I'm not trying to undermine your suggestion, because I applaud what you're doing and how much gusto with which you're doing it! I just wanted to offer a different opinion based on how the average employee or young manager (and I swear, I'm actually a very conscientious employee) will handle customer complaints that they see as "unrealistic." I'm actually heading out to the Petco I submitted a complaint to about a week ago, just to see if they're treating their fish any better. So it'll be a good field test... It's definitely not to look at another girl for my sorority. Why would you get that idea? *shifty*
But on a more positive note, good job deciding to pick a rescue! They're very rewarding, because you can look at the fish in the bowl and say "it would've died without me!" Or something of the sort.