A long time ago someone asked why their fish floated and gasped at the surface after water changes. I couldn't for the life of me remember why, though I knew it at one time, because I had once had this. I remember why! Its because of Hydrogen Sulfide poisoning. If you have gravel in the tank that is filled with gunk and you clean the gravel. The anaerobic bacteria releases HS into the water and the fish have trouble breathing and it causes floating. The cure is to keep very little gravel in the tank that is meticulously siphoned out every week, or get rid of the gravel gradually and replace it with flat river stones spaced out. Sorry my "senior moment" lasted so long!8)

Best wishes,
Kiko