Diatoms ... aka Brown Algae is common in newer tanks and really comes down to excess silicates in water, usually from PWC tap that comes from groundwater. Usually the BA goes away on it's own after a couple of months as the silicates are used up, but if your PWC source is silica rich, you may never truly get rid of it. I've had BA since the 1st month my tank was set up with no sign of it going away.
It gets annoying and a bit unsightly, but it relatively easy to wipe of. Switching to RO/DI will help to rid the silica's, but you'd have to recharge the other minerals you'd lose. Although you may not want to buy fish to deal with algae problems, Otto's love the stuff so that is another consideration.