Live feeder fish should only be offered occasionally as a treat. If you feed fish the same food all the time, and in particular live food, they regularly stop eating (go on a hunger strike) when their normal food is not available.
You should feed all fish a varied diet consisting of dry, frozen (but defrosted) and live foods.
They will eat prawn, fish, squid, mussel meat, fish pellets, flies, mosquitoes, moths and most other things that fit in their mouth. Get a few different foods and offer something different each day.
With big predatory fish, it is usually cheaper to visit a fishing shop and buy a kilogram of prawn, some white bait, blue sardines and maybe a few small squid from the freezer. keep them in your freezer and just defrost them as you need them. Have a few types of dry pellet foods to feed them in the morning before you head out to work, and use the frozen (but defrosted) foods in the evening.
When feeding prawn, defrost them, remove the head shell and gut (thin black tube in the body) and throw these bits away. Use a pair of scissors to cut up the remaining prawn tail and offer a few bits at a time.
When feeding blue sardines, white bait, squid or any other meat, defrost them and cut them into bite size pieces. Offer a few bits at a time. Remove uneaten food after feeding.
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If you are breeding fish or growing up young fish (a 2 inch Jack Dempsey is a young fish), and the filter is established/ cycled, you feed the fish 3-5 times per day. You should do a 75% water change and gravel clean the substrate several times a week to compensate for the food going into the tank. You do enough water changes to keep the nitrates below 20ppm.
When the fish have matured (around 6 inches long), you can reduce the feeding to once a day. You can reduce the water changes and gravel cleans to one a week too, assuming the nitrates remain below 20ppm.
When the fish are full grown (12 inches), you can feed them once a day or once every couple of days and do water changes each week to keep the nitrates below 20ppm.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.