Just an observation, Otos et al are often very busy on plant leaves that are strong, healthy and have no obvious algae.
This is something I've noticed many times and been intrigued by it. Yesterday, I was chatting with an ichthyologist who confirmed what I had long suspected, which is that Otos (and others) feed on something known as periphyton. Please take a look at:
We, as aquarists, would probably refer to this as biofilm. But, according to Wikipedia, biofilms can take many forms - including dental plaque! If interested, please see below:
And it's now an even wetter, rainy day.