Marine mix and cut it into smaller pieces for the fish. It contains prawn, fish and squid). You can also buy frozen raw or cooked prawn/ shrimp and keep them in the freezer. Take one out and defrost it. Remove the head, shell and gut (thin black tube in the body) and throw these bits in the bin. Use a pr of scissors to cut the remaining prawn tail into little pieces and offer a few bits at a time. Feed until the fish is full and then remove uneaten food.
Brineshrimp and Daphnia are available as live and frozen foods.
Mosquito larvae and adult mozzies can be fed to the fish. Just make sure they aren't sprayed with anything.
If you have a garden you might find aphids on some plants and these can be collected in a bucket and frozen or fed live. I used to have roses and would put a 2 litre icecream bucket under the new rose bods and tap the aphids into the bucket. When I had enough aphids I put them in a ziplock bag and popped them in the freezer for later use.
Small flies and moths can be fed to fish, again though make sure they haven't been sprayed with anything.
You can get wingless fruit flies from universities or online and grow your own in jars. If you get too many, freeze them.