It sounds to me like you need either a custom DIY light, or a single tube T5NO light. You can sometimes find the T5 with Blue LED's built in. You could also use CFL's.
Watts per gallon is completely useless these days. PAR is what you should be concerned with. Unfortunately, it is often difficult to get PAR data. The trouble with Watts per gallon is that 3 watts from a standard T8 is different than from a T5 or metal halide.
Hoppy on here has a great thread listing different PAR values from different lights, it is not a complete list, but should give you an idea of what will work.