Get some good bulbs and grow some algae. Or ...
There are plenty of good frozen veggie foods to choose from. You can search making your own veggie based food which is a great way to feed fish. You should also include some kind of protein maybe 3 times a week. Here's an awesome brine shrimp recipe.
Enriched brine shrimp I purchase two Oz. of live brine weekly. I place 1 oz/gallon in individual milk jugs and fed with O.S.I. Artemia food(algae meal, wheat flour, vitamins) and American Marine's SELCON food booster(1 ml/gallon). I add two drops Vita-Chem to brine before feeding