In my 15 gallon freshwater planted tank, I have 4 dwarf neon rainbowfish. While 3 of them are healthy, I have noticed one of them looks fat and has raised scales. It occasionally schools with the others, but mostly hangs back. The other fish are not affected. The tank has been set up since September 08. The temperature is 77 F, Ammonia 0 Nitrite 0 Nitrate 10. I do a 50% water change every month (planning to do 40% bi-weekly). I test with test strips. The last time I bought fish was one and a half weeks ago when I bought 2 praecox rainbowfish to give my existing rrainbows more company. I noticed on of them was fat, but it didn't look like bloat and I thought it was just constipated or ate too much at the last feeding. I use a biowheel filter. I feed my fish flakes and pellets every other day. How do I treat it?