People and additives...
I dont want to be rude, but you got a fish tank and you have to do husbandry on it. Most hobbiest love to do tank husbandry. While tanks are pretty to have, they take a lot of time to care for.
There is not an additive that you can use to avoid algae in a fish tank. The lights cause algae growth, not to mention the food you put into the tank for the fish.
Go buy a clean up crew and a mag-float. The clean up crew will eat most algae and what is left on the glass you can usually just brush off with the mag-float. You dont even have to get your hands wet.
The clean up crew I recommend should be turbo snails, bubble bee snails, red-legged hermits and some other hermits, a sand stifting star fish, a lawn mower blenny and maybe an emerald crab. I hope this helps.