Only from the experience I have with lentils, I would be a bit on the cautious side when feeding. Lentils tend to make us people "gassy", just like beans and brocoli, and I would imagine it could do the same thing to a fish. Now, that might be perfectly fine with a streamlined comet or common, while a fat bodied fish like a ryukin and its cramped intestines might run into a problem after feeding, and you would see the poor thing floating around for a few hours afterwards.