In the wild Piranhas have a widely varied diet, and in the aquarium they should be entitled to the same. By giving your Piranhas varied diet you can help prevent dietary difficiecies and health problems. Here I'll include the kinds of food that can be used to feed your Piranhas, and the kinds of food that should be avoided, as well as how often Piranhas should be fed.
Some people believe that giving your Piranhas live foods is detramental to their health. However, this is not entirely true. Some live foods are harmless and some can carry disease. Its really all about where you get your feeder animals from. Sometimes they are raised, or maintained under unsanitary conditions, and this is usually the reason for their disease. Live food is not absolutely vital to a Piranhas health, but it does ad variety to their diet, and Piranha seem to relish living food more than anything else.
The most common feeder fish used are goldfish and guppies. These fish are usually kept in crowded tanks under poor conditions, this is usually the reason why they are notorious for disease, parasites, etc. If you would like to feed your Piranhas live fish, pick out the most lively ones that group together when frightened (if they keep their distance from one another it means they could be sick), make sure there are no signs ich, or any other external parasites, and avoid the lethargic ones. After picking out the feeders you should quarantine them in their own tank for at least a week, while treating them with disease medication as a precaution. You should also keep the feeder fish fed on flake food during the quarantine period to keep them healthy and nutritious. A much more safe alterantive is raising your own feeder fish. Ghost shrimp are sometimes offered as feeders, these should be quarantined as well. Brine shrimp can be used to feed juvenile Piranha, they are much safer than fish and do not require to be quarantined.
Some Piranha Keepers like to feed their Piranhas live animals, such as living mice (usuaully baby ones). This is not a problem, so far I have not heard of any ill effects from feeding them this type of food. I think they should only be fed "pinkies" (baby mice) though, because they are hairless. A mouse with its hair can possibly cause an intestinal problem for your Piranha. If you would like to feed them larger mice, try and find the hairless kinds. But be carefull though, a live mouse can fight back and bite hard, possibly injuring your Piranha. I have fed my Piranhas mice before, and I have no had any problems. One other thing to keep in mind; feeding them mice is no where near as messy as one would think, its about the same as feeding them large feeder fish.
Crickets, mealworms and other insects offered as reptile food can also be fed to your Piranhas, only buy the farm raised ones from your local petshop though, or raise them yourself. Insects collected in your yard can possibly contain pestcides, which can kill your Piranha. Mosquito larvae can also be used to feed juvenile Piranha, but its best to raise your own, since most mosquito larvae is found in dirty water. You can do this by filling a clean bucket with water and putting it outside in direct sunlight, within a few days algae will grow and mosquitos will lay eggs in it, and then you will have plenty of larvae to feed your juvenile Piranha.
Worms are the most safe live food you can give your Piranha. They are virtually disease free and readily available. Do not dig them up out of your garden though, as they could possibly contain pesticides. Worms can be found at any baitshop and are very cheap.
Frozen food made for fish can be found at almost any petstore, they can also be found in all baitshops. The process of freezing the food destroys most parasites and organisms which makes it safe for your Piranhas consumption. This type of food should make up the bulk of a their diet. Always thaw the food first before feeding.
These are the most common foods offered as food for your fish. They usually come in: brine shrimp, daphnia, beef heart, shrimp, etc frozen into small cubes. These are good for feeding juvenile Piranha since the foods offered are most often small frozen fish.
Frozen Bait fish:
Frozen bait fish can be found at any baitshop and most petstores, usually the types offered are: squid, anchovies, mackeral, and minnows. They are very cheap from the baitshops (costs less than the ones from petshops, and you get more) and great food for your Piranha. Some of the frozen bait fish offered at baitshops are scented/flavored though, this could possibly be harmfull to your Piranha, so ask for the regular ones.
Manufactured/Freeze Dried Foods:
Flakes/Pellets/Sticks, and Freeze Dried foods make great supplments to a Piranhas diet and are disease free. However they shouldn't make up their whole diet, especially the manufactured types.
Some of these foods are good for raising small juvenile Piranha (but like I mentioned it should NOT make up their whole diet) Most are fairly low in protein and calcium, and usually contain more vitamins than anything else.
Freeze Dried Foods:
Freeze dried foods usually come in: krill, plankton, brine shrimp, daphnia, white shrimp, tubifex, and blood worms. These are very high in protein, vitamins and other trace elements. They are best for raising juvenile Piranha, or as a supplement for adult Piranha.
Fresh Meaty Foods:
Meats and fish/shrimp found at your local market are also good for a Piranha diet. All of them should be frozen first before feeding to kill most bacteria and organisms that could possibly cause disease to your Piranha.
Fish and shrimp make up a good part of a Piranhas diet, its an ideal food for them. The shrimp is high in calcium and their shells are rich in carotene which is good for enhancing your Piranhas color. You should remove the head and tails (if any) from the shrimp before feeding.
Beef, beefheart, etc. should make up a small portion of your Piranhas diet, since most are high in fat.
Foods To Avoid:
The following should NEVER be fed to your Piranha: hotdogs, packaged ham/turkey/bologna/sausage/bacon/etc, fried chicken, fast food: I.E. hamburgers, etc. ALL of these contain some type of preservatives and spices, which can be harmfull to your Piranhas health.
How Often To Feed Piranha:
Juvenile and subadult Piranha should be fed 2 to 4 times a day every day. Adult Piranhas should be fed 1 to 2 times every other day. Feeding times can vary depending on what you feel is best, though. This is what works for me.