I agree that your tank is too small for otos, so might I suggest amano shrimp? They did a great job curing my 10gal of the Diatom Plague. It's easy enough cleaning diatoms off rocks and glass, but I had the worst time getting it off my plants - especially the slower growing ones. I've heard some people say they won't touch algae, but I added three amanos and they picked every single leaf clean of diatoms in just a few days. It was pretty awesome!
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