They maybe getting enough from the tank. remember food is not always plentiful in the wild and they are scavengers and will eat algae in the tank if hungry.
I Have 5 Clown Loach In a 30 Gallon tank. I Feed them freeze dried blood worms, flaked and pellet food and most inportantly cucumber! Chop the cucumber in slices and put on a weight and in about 5 minutes the inside of the cucumber will be gone! Its amazing!