My opinion: I love powerheads! Great for the fish, and great for the tank's environment. Water current is an absolut must, the more air the fish get the happier they are. if you are going to use one don't bother turning it off at night they are indeed virtually silent. Although how will you be using it? Because they suck everything through them including fish so make sure to put a filter on the bottom of it so it won't get plugged if you're not using it as an undergravel filtration unit. It also sounds like your fish are a bit young, give em some time, i almost think that its a good thing that they aren't tearing around the tank nipping at each other. I have kept piranha for close to ten years and the only time that they are truly active is when they are eating or being introduced to new tank mates, really they are a secluesive fish, even with a lot of current.