You need to know a little about photosynthesis. If your tank is getting enough light and you are still having trouble with brown algae, then the phosphates in the gravel must be quite high. Brown algae is actually quite detrimental to your fish, because it is consuming oxygen instead of releasing oxygen. No algae eater will touch brown algae. What the main misconception about algae eaters is they eat algae is not strictly true. They have rasps in their mouths so what they do is rasp this microlayers of algae from the surface algae grows on. You will notice that they generally will consume only green algae. That is because they are after the chloropyll
within the plant. Like any herbivore, they do not have the mechnisms in their gut to digest cellouse which make up the main stay of any plant. So yes whilst algae eaters eat algae, they are pleased with other foodstuffs offered to them, including believe it or not carnivorous food.
Phosphates can be removed by proprietory resins, but if it is phosphates that's causing you this problem, you need to eliminate the problem.