When I kept FW
fish I would buy beef heart from the grocery store. Trim the fat and tough parts and run it through a food processor where i could control the size for the fish I was feeding. Then put it in a freezer bag and flatten it to about 1/4 inch then freeze it. At feeding time I would break off a piece, defrost it and feed to the fish.
Another variation is to freeze it in large flattened chunks and cut it up while it's still frozen. This works well with larger fish.