Gar buoyancy issues

[John2708]

Hi so i bought 2 florida gar today to add to my group both 17 inch both seem to have a buoyancy issue swim tail up head down they can level out but its on and off there still young and i have a theory on whats wrong but wanted some input so the previous owner only ever fed them prawns nothing else has anyone experience this and found a remedy im thinking a better diet for a start ? open to suggestions thx