I have noticed today my female acara is spending a lot of time on her side. She will stay in place on her side, slowly straighten up, maybe pick up and dump a few mouthfulls of gravel and then slowly swim to another location and repeat. physically she looks fine, no sign of injuries or anything. Just keeps going sideways.
No other fish in the tank are doing this (the male acara, 2 salmontail catfish, a silver shark and a spanner barb) The tank is a large 4 foot (is higher and wider than a standard) I have not changed their diet lately and I topped up their water 2 days ago (maybe about 5% fresh water added to what was already in the tank)