My fish Posiden is having a bad case of fin rot that just won't go away.

He has had it twice now but the second time it came back a little stronger but wont get any better.

I'm keping it simple for now and doing frequint water changes and puting in aqurim salt. Some times I'll put in some "Betta Revive". (It's this stuff that makes the water blue) He now has fins that are quite short but it hasn't spred to his body yet. And im doing my best to keep it that way.
I am wondering what could have caused this.

I am keeping him in a small tank for now to make it easer to do frequint water changes. He is uasaly in a 1.5g tank witch has a mold (i think its mold

) problem. There is white fuzzy stuff in the tank wich happend the last time he had fin rot. Back then i thought it was just some little pice of papper towl that got in there but it came back and was all over the filter, heater and back wall of the tank. I don't know if I sould clean it out with sope and rince it real well or if i sould just get a new tank.
Sorry for such a long post. All comments are of value.