MH over a 20 gallon...

[midgetwaiter]

I think you need to take this a bit further and think about what kind of bulb you want to use. A 10000k 150 watt HQI would work just fine for most things provided you are either happy with the colour of the bulb by itself or you had a way to add actinics.

As soon as you think to yourself, "I'd like to try a 14k bulb in there" you're screwed as far as crocea clams or demanding SPS goes. You will see a drop in the amount of light in the correct wavelengths for photosynthesis (measured as PAR) the higher up you go in colour temperature. The drop in PAR between a 10000k bulb and a 14000k bulb can be nearly 50%. If you want to use a bluer bulb you will need the 250w.