Otos add a little bioload, plecos add a huge amount of waste and are a huge strain on any bioload, even BN pleco. Plecos are one of if not the messiest none predatory fish in the hobby.
Lack of Circulation is one the of the biggest causes of diatoms. I added a cave once that was directly in front of the outflow of my filter and it caused diatoms in less than a week where the cave was against the glass.
I have also seen tank that have been set up for years that get diatoms. The theory is all the beneficial bacteria and the "slime" that acts as a barrier to the silicates in the glass and substrate dies off and exposes the surfaces again due to lack of circulation most of the time. Unfortunatley some of the best sources of silica are tap water and substrate. Fake plants can be a major source depending on how they were manufactured.