ATI powermodule over 90 gallon
 

Do you guys think a 6 bulb 54w ATI Powermodule T5 fixture would be enough light for a 90 gallon thats 4ft Long 2Ft High ? For growing sps mainly?

For comparison I have an 8-bulb T5 Tek-light over a 120, same depth and length but less width of a tank making the 8 bulbs roughly comparable to your 6 bulbs. The Powermodule is said to put out more PAR than the Tek-light. I can grow easy SPS (montis) on the bottom of my tank and haven't tried harder sps. So I would say you could probably grow SPS with your system but probably not as well as an 8-bulb Powermodule or metal halides.

Sure you can. There are many people that have great success with 6 bulb T5's and SPS :)