I've heard of people replacing HQI bulbs after 6 -12 months of use, then others change there bulbs when they notice a change in the color spectrum. It's all up to you though. The older the bulb gets, the chances of it causing algae problems increases.
yeah thanks showtime. i am pretty sure you have responded to all of my threads so i appriciate that a bunch. Thanks very much. Man those bulbs are expensive!!!! also is 20,000k better or is 10,000k better?
No problem!! HQI bulbs are pretty expensive. I'll be replacing my halides next month so I feel your pain. 20k bulbs have a lower par rating than 10k bulbs, so I would say the 10k bulbs are better. 20k have a blue look to them and don't require supplemental lighting. 10k have a white look to them, so most people supplement them with actinic lighting to get a blue/white look.