If your tank is 60" by 18", I'd consider running 2 parallel with the tank length. Use the PFO parallel reflectors, not the perpendicular ones. Sanjay Joshi did a study on these reflectors; if you're using 2 over this size tank, you need to light an area 18" by 30" with each bulb. The parallel reflectors have a light spread that's closer to this (rectangular) than the perpendicular ones (closer to a square) If you use more bulbs, like 3 over this tank, each bulb needs to light a 20" by 18" area; for that the perpendicular reflectors would be better.