Sorry, Watts Per Gallon doesn't work for T5HO bulbs. Please refer to the lighting sticky, BBradbury.
Specifically, the lighting with PAR
It discusses the "myth" of WPG.
Originally Posted by BBradbury
Even though the watts per gallon rule is a bit outdated, it can be a step in the right direction to getting you to the right lighting. 6500K bulbs are recommended for planted tanks. These come closest to natural daylight at 5500K. The color is light blue to the eye.
If you keep to the less demanding plants, then 1.5 to 2 watts per gallon, even using the watts per gallon rule, puts your tank into the moderate light range. This amount will grow more than enough plants and a wide variety to fill a 75 G tank.
CO2 isn't necessary unless you get into the more demanding plants. When you increase the light, you tell your plant to grow faster. In order for the plant to do that, you need to increase the food accordingly.
A nicely planted tank isn't difficult. But, you need to research the plants you want and then give them the proper lighting, nutrients and pure water conditions.
Just a couple of thoughts to consider or not.