Recurrent fin rot

[knifegill]

My water is A0, Ni0, Na10 and I have no tests for hardness. Can hardness be stressing my cories out to the point of rot succeptability? I have smooth, round gravel of the pet store variety, multi-colored (before I was into more natural appearances). Sanded slate and driftwood decorations, with a recently added stalk of hortwort. I change about thirty percent of the water, treating new water with Prime conditioner, once a week. 79-81º. Two HOB filters provide medium current.

It began with a misadventure involving a Green Spotted Puffer. The cory who was bitten worst has had missing fins since before treatment with triple sulfa. When I moved them to a 32 gallon from a 10 a few months later, the gimpy one's fins grew back a little bit. This was only three weeks ago. Now they are gone again and the caudal has joined the disappearing game with a slice down the center, almost reaching the body!

It has been over four months since the puffer incident and there is no detectable reason for the sickness to persist. My cories are fed six different types of food and are active and happy in a group of six. They are albino Aeneus from what I can determine.

I would isolate and treat but that will get me nowhere. There is something causing this problem to persist and I must determine what it is before all my cories are corpses instead.

Update: Now, early this AM, the cory in question has almost no finnage whatsoever besides anal, adipose and caudal, and is constantly swimming upward while the other five rest on the bottom. I think I have no choice but to quarantine him in consideration for every other fish in the tank. I don't know what this swimming upward activity is prelude to, but certainly don't want it to happen to everyone. Please let me know what else you need to know to help me. I've attempted to fill you in on all known fields.

Irrelevant but curious: 32 gallons= 3x2=6 6 cories. 6 foods. Could it just be evil? Nah. But odd nonetheless...