I use 2 -250W MH over my 4 foot, 75 gallon tank. A typical 30 Gallon should be fine with 250 watts. What is important is the depth of your tank and what kind of coral you plan to keep. Though 250W in a 24" tall tank sounds like enough to keep SPS corals. I believe you might want to mount it about 10 inches above your tank, any closer and you might get a little too much heat, of course with no canopy you have a better chance of keeping the water temp down where it should be.
Here is an example for you, I have a 12" deep 30 gallon breeder tank, and I keep SPS frags in it, I'm using a 150W mh. at about 8 inches above the tank. Everything is growing quite nicely.
Sometimes there are topics that have been discussed to death, and you get little to no response. Try the search in the early morning or late evening and you will probably be able to find several links on the topic. I was asking about tempered glass, and all the responses suggested I do a search. Theres lots of good info out there, sometimes you just gotta be patient and look around.