A standard 75 gallon tank is only about 20 inches high. At 20 inches one T5HO bulb, with a good reflector, gives you about 60 micromols of PAR. That alone is too much light to use without having very good CO2, good fertilizing, good water circulation in the tank, good water surface ripple over the whole surface, etc. Since the tank is 18 inches front to back depth, one bulb can't do a very good job lighting the whole substrate, so running two widely separated bulbs works best - like the 2 outer bulbs of the 5 you now have. Then if you raise the light fixture 4-6 inches, you can reduce the intensity down to around 40 micromols, get very good uniformity of light intensity, good access for reaching into the tank daily to pick out dead leaves, other debris, fluff the plants, etc., and you will have far less difficulty with algae, and more wiggle room if you aren't a perfect tank maintainer.