Just what I tend to do...
I walk in and see a tank full of dead goldfish, I make a face, and ask the owner(worker or whoever is there) "What's with all the dead fish?" And see what they answer. "Eww... why is there a dead rat in there! That's so gross, you can't think a snake is gonna eat that! Ugg."
Sort of clueless with no mouth filter, but non-confrontational. Saying the same sort of thing to other people in the store. In real life, I could butcher rats all day long, and it wouldn't bother me. In a 'pet' store that has a dried up mouse in the corner, uncleaned for weeks, I made faces, and act totally squeemish. It at least makes the owners think "Geez, our displays are turning off this possible customer" as opposed to confronting them which makes them think "Oh geez, some nutcase.. ignore them"
But again...... it's only my method(unless I see something that needs IMMEDIATE correction, which although nasty, dirty cages is something that most animals can live with temporaily)
When you leave, call anyone you think might have authority. Especially the F&W folks. Especially if they had bluegills, and the officers will hopefully tell them to clean up the stuff. An occasionally dirty cage is one thing, people can be ill and get behind on cleaning. But all the fish tanks being filthy, the filth in all the other cages you describe is just someone being lazy and not cleaning up.
Wolfy