To me, that is a lot already. In 15 years, over 40 species, and hundreds of fish, I have only medicated maybe 6 times. I did do salt baths and metro soaked food for wild caught fish coming in, but as a preventative. I never saw any of them actually have a disease or parasite.
I think 2 of those happened during quarantine periods.
I really keep coming back to your original statement that the fish would act like this after each water change. I am thinking there is something in or missing from the source water.
Yes, this bothers me...
I'm not positive about Petsmart, but most large box stores get their fish from one or two farms for the entire chain.
I've spoken to one of people responsible for getting the fish at this particular petsmart and they use there own supplier. That would mean if it's the breeders fault, all the female bettas all across the country would be getting tumors after one year. Bettas will like it a little warmer because the air in the tank stays more humid and they like that. I didn't see you mention Bettas before though so...
This tank use to house my betta sorrority (I think I've had about 3 different ones in this tank, but not all at once. Sadly the last one died of - possibly of tb - recently. Now I just have 2 boys and girls left in different tanks.)
Are you sure there is chlorine in your water? I don't know what the long term effects of a chlorine remover is when there isn't anything for it to react with. You can find out if you call your water provider and see if there is anything added. They should be able to tell you exactly what all is in your water and where it comes from. Ask them about heavy metals.
Yes, I'm sure there is chlorine in the water, the amount varies though, sometimes there is hardly any, other times it smells like a pool in the house - when someones running water. We got the water um, thingy?, lol, in the mail fairly recently, all kinds of nasty stuff in there (which might also explain somewhat why when I don't drink only filtered water it makes me ill. I wouldn't be surprised if there is something in there hurting my fish.
) There are heavy metals, but the api is suppose to 'neutralize' (?) heavey metals, or so it says. Is that not good enough?
See if you can't get some liquid test kits for your other params.