powerheads: when they start to look oogy. My powerheads grow some terrific algae
pumps: when something doesn't sound right. I can usually tell when something is up by the harmonics. Usually its sucked up some sort of algae or caulerpa
skimmer pump: seems every week. I don't know what is up with the stupid thing it seems to have a lull in making bubbles about every half second or so. I'm not as impressed with it as when I first got it (sedra something on a Euroreef ES6-2)