fin rot on 1 fish in brackish tank. HELP!

[flipz]

Ok, I have 2 Black fin sharks (technically columbian shark, or columbian cat fish. they are only like 4 inches long) and have had them for about 6 months now. I just moved around a month ago and now every single time I add water to the tank, within a few hours 1 of them has completely shredded dorsal fins, side fins, and her whiskers fall off. This never happened in my old place. I am doing the proper treatments to the water however it is still doing this. After she gets sick I immedietly seperate them (because the healthy, larger one starts to eat her!) and within a few days to a week she's back to normal. And this has never effected the other one. I've done 3 treatments in the last month with Binox but she doesn't seem to get better any faster, and it hasn't stopped her from getting it again.
All I can assume is it's something in the water down here. What can be done about this? Can I buy distilled water at the store and use that? Any suggestions at all? No one at any pet stores has been any help to me. They just keep trying to sell me medication to fix tail rot. Thanks in advance.
Brent

 

[beth]

Yes, I'd try the distilled water and see what happens. How long are you taking to mix up the salt? It should be at least 2 days under a good powerhead.
Personally, I don't know anything about these fish, but I'd also be sure that you have the salinity correct as well as water temp of the change out water. pH, perhaps, also needs to be checked since you're have a brackish tank. Be sure that you know exactly what these fishs' water condition needs are and adhere to them.