So sorry about your fish! It's sad that good people like you have to clean up and try to fix the stores' shortcomings!
You should take those photos to the manager and ask what the heck is going on with their employees that are supposed to be caring for the fish to allow the water to get to such a horrendous condition. And ask the manager if the employees aren't doing an adequate job caring for the fish, why HE or SHE isn't doing an adequate job in being a supervisor! Stupid rats!
ALSO, send the photos to a regional manager, and the regional manager's manager... etc. You get the idea. Nobody is going to change their practices unless there are consequences to them making $$ or unless their job is on the line. And no reason to change if no one is complaining.