lagger: I believe one of your underlying questions remains unanswered.
You asked, "Is one 250w enough for 36" x 18" x 24"?" I believe the answer to that question lies in both reflector and height from tank.
My 75g is 36"w x 18"d x 28"t (four inches taller than your tank). I have elected to go with two bulbs to get end-to-end coverage, among other reasons. I will be running 2 x 250w DE Phoenix 14k (unless the 'saki's are out by the time I am ready to run water in the new place...) on PFO M80 Ballasts (just shipped and will arrive on Tuesday!!).
If you are comfortable with light seep and build your "reef" appropriately, then one bulb can light that tank with certain caveats. A significant amount of light will escape over the ends of your tank. A noticeable amount of shadow will be created at the ends of the tank. Now, this is perfectly fine if you place your corals to maximum advantage. As many have stated, certain reflectors are going to do a better job of getting that light well-spread. A DE bulb on either an M80 or E-ballast is what will optimize that single light for that tank.
HTH!
(Your mileage may vary; certain rules and restrictions may apply...)
You asked, "Is one 250w enough for 36" x 18" x 24"?" I believe the answer to that question lies in both reflector and height from tank.
My 75g is 36"w x 18"d x 28"t (four inches taller than your tank). I have elected to go with two bulbs to get end-to-end coverage, among other reasons. I will be running 2 x 250w DE Phoenix 14k (unless the 'saki's are out by the time I am ready to run water in the new place...) on PFO M80 Ballasts (just shipped and will arrive on Tuesday!!).
If you are comfortable with light seep and build your "reef" appropriately, then one bulb can light that tank with certain caveats. A significant amount of light will escape over the ends of your tank. A noticeable amount of shadow will be created at the ends of the tank. Now, this is perfectly fine if you place your corals to maximum advantage. As many have stated, certain reflectors are going to do a better job of getting that light well-spread. A DE bulb on either an M80 or E-ballast is what will optimize that single light for that tank.
HTH!
(Your mileage may vary; certain rules and restrictions may apply...)