There could be a couple reasons while your getting brown algae-
- Rocks or sand containing silicates and silicon
- Excess waste in the water
- Still or stagnant water
- Hard of mineral rich water
(The rest is under the read more, this reply is quite long!)
Changing out your rocks or sand could help. It should say on the back of the package somewhere in the tiny writing if it has silicon or silicates in the rocks or sand.
Getting a filter for your tank might help if you don’t have one already. It would get rid of all the excess waste and it wouldn’t leave the water totally still or stagnant constantly.
Another way to prevent the algae is to not have your tank in the light constantly. You could also use a different water for your tank.
Brown algae is nothing to be afraid of all it is, is a CO2 guzzler but I wouldn’t keep it in your tank. I would try some of these to see if the algae gets any better. I would also clean out the tank again to rid of all the brown algae already in it.
I hope this helps and you get the algae to go away!