Then keep the lights you have already, and add co2 through DIY or a Hagen system. Don't spend 200 plus bucks for a pressurized system since it's only 20 gallon. Use florish for your trace elements. You can spend the extra cash on plant fertilizers, or do what I do.
Use stump remover (Potassium Nitrate) to add nitrate. Use Epsom salt to add magnesium, use Nu-salt/No-salt to add potassium, and use Fleet brand Saline enema (yes, I said enema) for phosphate. Tons of planted tank people use those, and that's how I found out about it. That should cost you about 10-15 dollars and lasts a lond time. If you bought Flourish nitrogen, flourish potassium, and flourish phosphorous it would cost you about 30 dollars. Using stump remover, no-salt, and enemas may seem ghetto, but they contain pretty much the exact same thing as the commercial aquarium products. I do reccomend using Flourish trace though. I need a bottle of that myself.