I see, at one point I was considering piranhas but I wouldnt try it in a 75 due to the size and general inactivity
If you are thinking more than one piranha then it has to be reb bellies or caribe. Anything else has to be solitary or grows too big for a 75g. Unfortunately, it is a hit and miss with piranha. Some are extremely active and aggressive while others hide all the time.