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Could you get a picture of the catfish? That would really help in the identification of the catfish, and once we know what it is we can easily find out its dietary needs
Also, I want to point out that depending on the catfish, it may not control the population.
It's a syndontis catfish. Im not sure exactly what species. What population did you buy it to control? Anyways, they're not picky eaters. Sinking carnivore pellets suplimented with algea wafers and frozen bloodworm should round out a diet for you.
If you are worried about overpopulating your tank nature will take its course unless you have a very heavily planted tank few if any fry will survive. The parents will eat them