Goldfish fin rot

I am at my wit's end, if I ever had any at all. Those fantail goldfish that
I bought a month or so ago that immediately came down with both ick and fin
rot, even though all of the equuipment was new and never used.

The ick is long gone, but the fin rot keeps coming back. I keep changing
the water and putting in antibiotics. It disappears and then it's back.
Can't understand it.

I used to raise tropical fish and had over 20 tanks. Never had this kind of
problem.

Can anyone help me?