Brown algae (diatoms) can quite often be seen in new tanks because of the high silicate levels. Increasing the lighting period will sometimes get rid of it.
Try and get some reflected sun into the tank for about an hour a day and that will let some green algae start. As soon as you get the green the brown will disapear then stop the sunlight and the green will disapear too.
I had that type of algae all over in my 20 gallon tank and got some olive nerites. Within a couple of weeks the brown algae was completely gone (they don't eat plants either). I ordered them from azgardens.com.