Piranhas require quite a bit of training to get them to take anything other than live/fresh food, and it's never guaranteed.
However, it doesn't have to be beef heart, or the garbage feeders you buy at your
LFS.
You can start your own feeder tank by raising mollies, guppies, or swords (all are livebearers and breed like rabbits). The important thing is to keep the population down to healthy numbers (no overstocked tank) and to feed them high quality food.
In addition, you can try different meats and seafoods (piranhas eat mostly fish in the wild, so focus on fish). Try to find a small co-op or natural food store in your area and ask them if they ever have meats priced to sell quickly, or if they sell stuff that's been in their freezer too long as pet food. One co-op near me sells their meat for super cheap the day before it expires, and a specialty meat supplier in Hudson, WI sells stuff that's been in the freezer too long as dog/cat food ingredients. The stuff is a bit freezerburned, but he sells it for $1 a pound. Even if you cut out the freezerburned parts, it's still an excellent deal.
Aside from this, you can almost always get catfish and a couple of other kinds of farmed fish for pretty cheap. These could be the normal meal, with the others as treats to vary the diet and make it healthier.
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