Brown algae are hard to control because not many things eat it . Plecos will keep it at bay as they feed on the bacteria slime . If you have brown algae it is a good idea to step up your water change regime . Note that frequent weekly and /or bi -weekly changes are far better than monthly changes . In fact , changing the water very very often keeps the parameters stable . If you wait a whole month , there will be fewer nutrients for plants and Ph may fluctuate wildly .
Rain water is not a good idea at all . It is void of mineral salts and will not support your plants and other inhabitants. It can be used temporarily to induce spawning but the ph can plummit without the buffering of minerals .

I hav'nt used any chemicals in five years . No carbon , no anything. Just good old fashioned water changes .