Agree with the above. To clarify, you can use RO/DI water to top off your tank when water evaporates, and when you mix up your waterchange water. It will help.
Be prepared to go thru various algae phases, since your tank is a new set-up. It can take a few monthes for your tank to find it's equilibrium, do not despair! You will see brown algae, then hair algae, cyanobacteria, maybe some more brown...maybe some more green...etc...Just keep doing your maintenance, and things should balance out eventually.
Patience is a virtue. Well, that, and a cold beer now and then 8)