Well a good staple would be a pelleted food called Mazuri. Its the best commercial food out there. Then keep a constant supply of feeder fish (rosies) in the tank. Not only will he eat them, but they will help clean up the tank. You can also offer him crickets, worms, mealworms, aquatic plants like Anacharis and Romaine Lettuce.
Chris
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0.1 Red Eared Slider, 1.0 Common Snapper, 1.0 Bearded Dragon and a 55gal Native Fish Tank with a 6in LM Bass, 3 Crawdads and a Pleco.
I use to have a collection of Leopard Geckos that I bred, but have sold them.