Fish On A Diet?!?

[sforzauk]

Hi, I've got a red tailed black shark in a 58 litre tank. I've had for about a month and she's sharing with two opaline gouramis. She has been sucking on the panes, pebble and plants, which is eating (I think) When I feed my other two fish, they gobble up the food whereas she hovers. Today, she ate a discarded fish flake so I know she wants the food.I've tried pellets but the gouramis eat them all up before she has a chance. How can I get her to eat, or at least stop the others eating all of the food? Thanks for any replies