First, nice tank!
I can't tell what kind of plants you have, are those swords? If they are you don't need to go too much overboard.
For a low tech approach you could probably:
-Use fluorish excel (by seachem) as a source of carbon (acts like CO2)
- Seachem fluorish for macros and micros
-CO2 alternative: take a look at DYI yeast CO2 systems, this will be cheaper than fluorish excel but a lot more time consuming.
The high tech approach (its not cheap):
-I don't think you need this for your current plants but just for the sake of giving you more options
- This approach will let you keep a wider variety of plants.
-You will need to upgrade your lights to 2-3wpg.
-Dry fertilizers
-Pressurized CO2 system.
The key is finding the right balance. Light is the fuel, CO2 is the building blocks and macronutrients/micronutrients are like for plants what vitamins are to us. If any of these factors becomes limiting the result is usually poor plant health and probably algae.
I would recommend the low tech-approach with liquid fertz.
