Excellent summary of foodstuffs ma good man
If you don't mind me adding a couple things from my experiences.....
Carnivorous animals aren't 100% meat-eaters. They consume portions, albiet very small, of plant material found within the gut and intestines of their prey. This is referred to as "Gut-Load".
It is a very good idea to keep your feeders separate for a couple of days and feed them a good healthy flake food. Looking at the ingredients one can see some non-animal products contained within. These companies have done their research to determine the best mix for the best health of your fish. Let the feeders plump up with it until their stomachs and intestines are full. Then feed 'em to your fish.
Frozen food loses certain vitamins especially the B-vitamins and the food will need to be supplemented with a safe source if frozen food is a mainstay of the fish's diet. An easy way to do it, and one which works for me... err.. I mean Kilzses
, is to poke little holes into the thawed fish/meat and insert little pellets of sinking pelleted catfish food and pellets one would feed their hamster/rabbit.
The pellets are a complete food source and the advantage of hamster or rabbit food is that it contains many additional vitamins as well as vegetable matter such as alfalfa, corn, wheat, etc.
By poking a couple of these pellet foods in during every feeding you can rest assured the frozen food isn't missing trace elements and vitamins. It is very important especially if the piranha doesn't get to eat the organs, gut-load, and brain during feeding as it would in the wild but just chunks of plain fish flesh is deficient in some of these important components. An advantage is the roughness factor added the diet for good digestion. ........important to fish too.
I have been doing this for many years and Kilzses has great health and temperament with clear eyes and beautiful finnage.
.....and a nasty-assed attitude to go with it!!