Why are my neon tetra's tails deteriorating?

I have 2 neon tetra fish, and recently i noticed that their fins and tails are deteriorating, I have had them for 3 years and this is the first i have noticed it, the one fish is still swimming pretty well, however, the other is flopping at the bottom. I cleaned the tank to try to eliminate the problem because some white fluffy stuff similar to cotton was growing on the lid. Any idea of the problem and how to solve it?