I had a managuense not to long ago as well and I fed him a staple of live insects, meal worms, crickets, pond and bladder snails. He loved it. And grew really fast as well. Never showed signs of illness, and it promoted his natural hunting skills. Also you can feed rosy minnows when he gets old enough and eventually gold fish feeders
Would not offer feeder fish unless...I was raising the feeder's .
Store bought feeder's are often kept in crowded tank's ,poor water condition's = many sick feeder's,parasites,bacterial pathogen's.
All it takes,, is one sick feeder fish to bring these issues to otherwise healthy fish once your fish ingest's the sick fish or fishes.
Thiaminase content of feeder's can also present vitamin deficency, and or fishes become easily bloated from too much animal protein's that digestive process can't quickly handle.
Better to offer bit's of krill,shrimp,in moderation, along with vegetable content from food's rich in them, such as New Life Spectrum for large fish.
Fishes will often refuse much more nutritional food's, once feeder fish make the menu. Can be hard after that to wean them off of them, for many folk's don't like fasting,or starving,their fish to try and get them interested in prepared food's once again.