Get some frozen brine shrimp or bloodworms ( if your Coreys are large enough to eat the worms).
Thaw the frozen food in a little bit of tank water.
Using a turkey baster, suck up the thawed food, aim turkey baster right in front of the Coreys, and target feed them. You can do this after lights out if needed, if the other fish are too greedy.
As long as your Coreys look fat they should be getting food.
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