I would discourage your from using excel in a 90 gal. Not only would you run out of money quick, but it is not the way to go. Buckle down and get pressurized CO2. It will be more than worth it to save you from impending headache.
4x65 is perfect for a 90 gal. You must dose ferts and run CO2 at about 30ppm or you will run into algae problems. Then you can sit back and watch your plants grow~
