It sounds like he's really hungry. Have you tried adding a slice of cucumber, carrot, or lettuce?
Floating plants might be a good solution. Something like riccia fluitans, water lettuce, water hyacinth, ricciocarpus natans, Ceratophyllum demersum AKA 'coontail' or 'hornwort', najas guadalupensis AKA 'guppy grass'. I think hygrophila difformis can grow floating.