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If you are feeding them a high protein food, try switching over to a vegetable or algae flake. I use a combination of 40% vegetable, 40% spirulina and 20% cichlid growth flakes. Kribs are primarily herbivorous, but usually prefer decaying plant matter, which makes softened flake perfect for them. If they are eating healthy plants then I suspect that they are missing something in their diet that the plants are providing.
I feed my Kribs a frozen shrimp salad as well as flakes. It consists of pureed river shrimp and spinach mixed with clear gelatin then frozen into cubes. They seem to prefer this over my live plants. I think it is because it is easier to bite than a leaf, and has the shrimp in it as well.