Why is oxygen important?
The shallow streams of the llanos are not only sunny and plant-filled, but also have high oxygen levels. At a pinch, Otos can gulp air, using their specially modified swim bladder as a kind of lung, but in the longer term these catfish really do need clean, oxygen-rich water in order to do well.
Don’t try to keep your Otocinclus in an overstocked tank with minimal water movement because they won’t thrive in these conditions. At the same time, don’t assume they’ll be happy in a hillstream biotope either – these are fish from slow-moving lowland creeks and rivers, not fast-flowing highland streams. Brisk filtration and moderate stocking are the keys to success, although it’s important to avoid very high temperatures too, as the warmer the water, the less oxygen it holds.