I forget how many watts the 4 foot T5's are, I think like 59 watts. Three or four of those would work great, you may even get away with only using two.
At the moment I have Thallasia growing under 80 watts of NO flourescent light, and its growing quite well even sending up new shoots. Thallasia is considered to be one of the more light demanding seagrasses. Certainly more light would be better.