What signs are there when piranha are sick?

Answer 1

"Danger: do not put any body parts in tank!"

But seriously, here are some symptoms of some common diseases:

Ich is a common fish disease, the symptoms are small white cysts on the fins and body of the fish. The fish will also try and scratch itself against rocks and gravel. It can be treated with medications from your local fish store.

another common disease is fin rot. it appears as clouding on the edges of the fins, and eventually becomes badly frayed and mangled fins (or tail). This disease should be treated immediately with a broad spectrum nontoxic antibiotic. medication is also available at your local fish store.