I have never heard of t6 bulbs. Basically the number portion of T5 stands for 5/8" of an inch in diameter. T8 are 1"(8/8") diameter and T12 (12/8") are 1.5" diameter. I would suggest 4 x 54W over your 75. I will dig out the link for where I bought my T-5 from tonight if I get a chance.