SERIOUSLY NERDY INFO AHEAD:
What sort of substrate do you use? Is it a NPT? If so, might have an issue with silicates. It's present in sand and natural gravel, and silicates are what feed diatoms (brown algae). Your tap water might also have silicates in it, which contributes to it. Hopefully it won't be a problem for much longer. You will find it mostly on the sides of the tank, some leaves, and it'll look like goldeny-brown circlish spots, and wipes off easily.
If it is real brown algae (or black beard/brush algae), it'll look hairy, either long or short. Then it's a combination of nitrate (up your WCs), phosphate (maybe, this is added to tap water and is produced by rotting fish food), and not enough CO2. You could always do flourish excel or something like that instead of a fancy Co2 system. Excel got rid of my BBA problem fast.
If the algae feels slimy and is dark brown or blue green, that's BGA...it's actually closer to a bacteria, and it's a whole 'nother ball game. I have this in my big tank and it's terrible.
So anyway, hopefully this helped a bit. I totally know how annoying algae is.
Beth, mom of two humans, and caretaker of eight cats and seven bettas. Yes, I'm crazy.