"Oh yeah, I have live plants. Does that mean there will be no algea for them to eat? If so, will they eat leftover peices of food?" - FeederFish33
Plants have no effect on algea, apart from it will grow on the plants.
"I say you know your fish well enough to try it. What is there to lose? They won't hurt your p's" - traumatic
Assuming you are talking about "Gyrinocheilus aymonieri" they could harm your piranhas, they are known for being aggressive when older, and also at any age they can sometimes get a taste for the slime from the sides of flat-sided fish - like silver dollers, anglefish, gouramies and piranhas (if they are not eaten first)
also when larger they tend not to eat algea, especially if other foods are available.