Depth is also a deciding factor but i took his tanks depth of 18" into consideration. Also most people put there halides about 8" from the water to have better access to the tank. That equals about 26" . The VHO
' are closer to the water but I am not a VHO
fan myself. The tend to get less intense in the middle as the bulb gets older. This is just my opinions and nothing is written in stone in this hobby. But 99% of the colorful SPS
tanks you see all have Halides over 175 watts.