Im assuming your already using gelatine in your mix as a binder, so you could simply add some floating fish pellets to the food mix during preparation to provide slightly positive buoyancy. You could also bind it to polystyrene or anything that floats too but the fish might eat some by mistake.
Alternatively, you could make a floating feed ring with onion skin netting and let the fish eat through the net? Or let it sink and make sure you have some scavengers.