Listless Goldfish

The saga continues....

I treated some GF with Kanaplex for what I thought was Columnaris and the
Cottony patches subsided, but did not disappear. A few days later they
started to develop Septicemia and I thought I was going to lose the largest
fish. So I'm treating them with Marycyn II now and they are looking better,
but they are not eating well and the middle fish is very listless. The
smallest fish never developed any symptoms and still seems fine. He seems to
have an immunity to whatever is bothering them.

I really can't explain what's causing their problem as the water conditions
are fine, in fact the Nitrates are lower than I would have expected. I think
it must be a parasite of some kind, but I don't see any indications of it.
Any plant/filter friendly recommendations for a precautionary treatment that
won't stress the fish too much.