Piglet, my lovely litle red Petco rescue,keeps having fin rot issues. It seems to have been a problem since I got him (I don't think he did much flaring before, poor little guy). I put him in his own ten-gallon tank at work, gave him a bit of medication, and his fins became gorgeous.
We reduced our hours in the summer, so I took him home and put him in a ten-gallon tank already inhabited by my little Spectra girl (with a divider, of course). He loved blowing bubbles for her! They would play for each other for hours, but they both had plenty of hiding spots for when they got tired.
About 2 months into Piglet's new home, I noticed the fin fungus coming back. I moved him to a hospital tank, dosed him, and no more fin rot. Within two days of being moved back in with Spectra, the rot reappeared.

Put him in the hospital tank again and tried a little sea salt and slightly elevated heat. No good. Gave him another course of meds and his fins clear up. At a certain dilution point after a few water changes, the fungus comes back.

Any suggestions? Those meds are way harsh, and I don't want him living the rest of his days swimming around in something that carries a cancer warning for humans. Plus, he and his girlfriend really miss each other. The hospital tank is right next to Spectra's, and they spend all day swimming next to each other.