You can try adding a otocinclus but if the tank is newly established then I would just suggest to do some extra water changes (10%-15% water changes). I would put 1 nerite snail and that should take care of the algae.
I would also say it depends on how you want your tank to look. Typically it should have 1-2 pounds of gravel for each gallon of water. So i would put 40 lbs of gravel in a 40 gallon.
I voted for oto's.
My oto eats all the algae off my anubias plants keeping them in great condition. I also feed him zucchini when i notice that he does not have a round belly.