Jungle labs "tank buddies" fungus tabs work well, especially for treating your whole tank. Dosing is easy, it's very light medication, and it does'nt wipe out your plants or snails. It takes days to work though, and you may have to treat more than once.
Pimafix is the big dog medication of choice for fungus, but as stated earlier, it's harder on the fish, the tank, and it's easy to overdose.
If I were you, I'd isolate the sick fish in a QT tank, and pimafix him. Meanwhile, I'd half strength dose the primary tank with jungle labs as a preventative measure to block or retard the spread. Fungus was an ongoing problem with my tanks for a while, I eventually found a spot in my substrate that was harboring it.
Good luck!