I use 4 T5's on my 40 long 36" x 12" wide and 4 T5's on my 29g 30" x 12" so 6 would be plenty on a 18" wide tank. I keep SPS in both tanks. And with the 40 breeder being so shallow I would think even a cheap 6 lamp fixture would work? Maybe you can get lucky and find a ATI power module used for a great price.