I got 2 Fantails a year ago, one has grown and is OK, however the other one hasn't grown much and doesn't seem to eat much, it also floats to the top as if its sleeping, its been like that for 6months. At night they both act normal but just during the day it doesn't swim much. They have a big enough tank and a filter. We had flooding which may of affected the water but I use tap safe and disease safe every time I give them a clean out (once every 3 weeks).

Its OK. I had some fish before. Ok here is what I think. I think you should buy another tank (if you can afford it) and separate the 2 fish. Then wait a week and see what happens. What kind of fish food are you giving them?