Ive had an 18w 50/50 PC light over my 3g pico and i was wondering how do u konw if it is guna die on you? is it like MH when it just dims, or like an Incendecent bulb that goes out fast.
You should be replacing the tube at least once a year. PC's often don't get noticeably dimmer, but they do go thru a color shift towards the red-end of the spectrum, which is why old tubes promote nuisance algae growth.