I recently purchased a MH fixture with a 150W MH and 2 x 14 w T5 actinics. I haven't installed it on my tank yet since I just got it in, but i have had thoughts that maybe a 150W MH is too much for my 18 Gal. Since the ballast is set up for 150 watts, does that mean i HAVE to use a 150 watt bulb, or can I use a lower wattage HQI bulb? I was thinking maybe I could just put a 75 watt HQI in its place if I decided the 150 watt was too much for my tank. Thoughts?