Most "algae-eating" fish will deal with diatoms (so-called brown algae). Otos were mentioned, a group of 3 minimum is recommended as they are a social fish much like corys. Another option if your water is soft and acidic is the Twig Catfish
; it attains 5 inches but is so slender that it is suited to smaller tanks, and it is not a swimmer so space is not significant; it just slowly moves from plant leaf to plant leaf, wood, rock, glass, substrate--cleaning off common green algae and diatoms. Another similar option for acidic to slightly basic water is the Whiptail Catfish
. The common species and the newer "Red Lizard Whiptail
" are both suitable in a smallish tank. The Bristlenose Pleco
is another, though this fish has much more of an impact on the bioload than the afore-mentioned species and I would not confine it in a 15g.
Have you looked into the cause for the diatoms? Normally this is only seen in newer tanks and once the biology has become established doesn't appear again unless there are silicates in the source water.