"can i feed raw meat to fish"?

[Cow]

beef, chicken, pork

or is it too fat?

seeing how they can eat blood worms and stuff and piranhas eat cows

[tattooedgemini]

you could but you will be adding different bacteria to the tank and possibly parasites and other contaminants(like salmonella)

[andyjh_uk]

Beef heart is often used to raise Discus it being hi protein has as been pointed out though the amount of cleaning required to keep the tank upto scratch sometimes defeats the object.
It all depends on the type of fish in question.

[ANNiE]

to a parana

[theironlung4200]

pork and chicken can be used for some speciesBUT) if you tend on using meat for your larger cichlid breeds than be prepared for a nastey messy tank.

[JohnZ]

I'm pretty sure Piranhas don't eat cows, as you normally don't see too many wandering around the Amazon River... Also keep in mind Piranhas can eat pretty much anything. You can feed most carnivorous fish those things, though I'd stay away from chicken. Not too much of that either, because too much protein causes constipation.