I'm the fish, reptile, mammal, and bird department manager at one of the larger Petsmarts in the company - we're #3 out of around 1,500 stores in live animal sales. I can confirm that we get all of our fish from the same sources that most of the LFS's around here get them from.
At our store, the main system gets treated with
Ich-X twice a day. We bypass the carbon block on our filtration symptom for 4 hours (the maximum time it'll allow us to), so while the fish don't get the full dose they likely need, it's better than nothing. If we see a fish with ich, all the fish in the tank get moved to a sick tank in the back, where they're treated with 1 tbsp per gallon of salt and occasionally Methelyne Blue. If it's just a bacterial or
fungal infection, we just move the affected fish to the sick tank. The salt will usually take care of it.
All of our
betta cups get treated with 1/2 tbsp per gallon of salt, which we've found to
significantly cut down on fish death and the amount of sick ones we get. They get their
water changed twice a week on Wednesday and Saturday or Sunday.
All of our tanks get completely deep cleaned at least once a week, and spot cleaned daily. We still get some diatom build-up, but I'd say it's rare for us to have a "dirty" tank. We use fake decor in most tanks, so we remove it all and soak it in bleach for a few hours, scrub it clean, and the soak it in water that's been dosed with a bunch of
Stress Coat. Once they no longer smell of bleach, they're put back into tanks. This happens once a week as well.
The entire department (including the other animals in the store and all the dry goods associated with them) is run with 3 full-time employees and a few cross-trained part timers. So, there's definitely no excuse for the poor quality chain stores you see. A lot of the basic care is actually right there in Petsmart's internal guidelines, so believe it or not, the information the store has (while dated in most cases) is actually quite good. It's all about how the manager runs it.