Water Crystals are not an acceptable food source but is an acceptable water source. You need to add some sort of protein as your food source. Dry dog/cat food etc. Some people use the apples or ornges or whatever as a treat one or two times a week.
I personally use apples and oranges as a sole water source at all times. And a mixture of gold fish flakes (this is very expensive though but has a lot of protein and vitamins and minerals AND I guarantee your feeders will love them) and dry cat food as the protein/food source at all times.