My guppy hasn't been looking so good for the past two days. He is always hanging around at the surface and isn't so active anymore. He also has a clamped tail fin and dorsal fin. Tank params are Ammonia-0 Nitrite-0 Nitrate-about 10ppm. The tank is 6.5 UK gallons and the only other fish inside is another male guppy who is perfectly fine. Anybody have any idea what is wrong with him and how to treat? Thanks.