How many watts do you have now?
Potassium, magnesium and possibly calcium for starters. Test your nitrates and phosphates. If they don't exist then you need to dose them also.
Green spot algae, phosphate shortage
If you had that good of growth before then why get more lighting? If the plants are getting leggy all of the sudden then it may just be old bulbs. If they are a year or more old then replace them before doing anything. Flourescents lose spectrum as they age and it could be an easy fix.
As for CO2, if there is a nutrient imbalance then CO2 will only make it worse. Figure out the problem and get the balance back before adding CO2. Once you have the balance and good plant growth again then do CO2 or higher lighting or both. Chuck's Planted Aquarium Calculator for Windows
Go there and download the dosing calculator. It will help with everything you need to dose. I dosed the recommended amounts for months doing 50% water changes every week and never lost a fish.
As for Excel, get a small pipete or syringe and dose the Excel directly on the black algae, it will kill it and quickly. Dose daily. I have never dosed double so I can't say there.