Mine use to go for my heater not my filter, but i've heard other people having the same problem with their piranhas showing aggression on their equipment. I wouldn't worry, it's just a sign of aggression. Just make sure they don't hurt themselves nor the filter. :smile:
Serrapygo and all else using glass heaters, I would recommend switching to titanium especially if your piranha is attacking it or accidentally crashing into it. Titanium will not(or at least should not) break if bumped into or attacked. My reasoning is the heating element being exposed to the water could cause much worse problems then just having to go buy another one. Electricusion (sp?) or a burning of the piranha if it rubs against it, and maybe more. And I agree with Wes in a possible solution would be to lower your temp to around 80 degrees...this should help lower the aggression.