My Betta is usually happy and healthy, spends time all over the tank, hides, eats well, blows bubbles... However today he started spending a lot of time at the top of the tank. He now looks like he has air trapped inside him somehow and is mostly floating on his side at the surface. When he tries to swim down to the bottom, it appears to get too difficult for him and he just floats straight back up to the top. His colour also seems to have faded slightly around the gills and mouth. Can anyone help me out with a suggestion of what this might be, and what I can do to help him?