Short horns do not require the quantity of ants that most other species do, and they are better served for the most part with non-harvester ants. BlueMax is the man to speak to if he will comment.
In the mean time, the tank you have is too small. A larger tank is needed to dissipate heat from the lights better, and also will make the lizard feel more at ease. Second, you want to get a bunch of small crickets from petsmart. They generally have two sizes. The large ones are not so great for horned lizards. You might also try a meal worm or waxworm but no more than one or two a day. The temptation will be great to give him a lot of these, but don't. Crickets are best. I believe you can get some non-harvester ants from antsalive. Look into that.
I hope this is helpful.
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