I suspect that you'd only need one of the 75 watt bulbs over a 15 gallon as long as it's a daylight bulb.
As an example I'm running a 2x40watt fixture over my 10gallon. One daylight and one 50/50 for a total of about 60watts usable light. I can grow most anything I want, and many of the plants turn shades of pink and red as they reach the surface of the tank. I've also got Pressurized CO2
on the tank and dose EI to keep up with the plant needs. I've had problems with foreground carpets growing up instead forming a carpet in this tank mostly because the stem plants grow quickly and shadow the rest. I don't intend to ever use more than 60watts.
With a 15 gallon tank, the WPG
rule isn't going to break down as badly, so you don't need quite as much more light as with smaller tanks. Further the VHO
bulbs are extremely efficent so a lot more of the watts are actually going to be used as light instead of heat. Unfortunately it is a bit of a guessing game with these smaller tanks, so until you actually try it you're not going to know for sure if you have enough light to do what you want.