I noticed that with a third oto in the smaller aquarium, the glass is quickly becoming crystal-clear again, the thin algae film across the back rapidly disappearing. There are a few mts in there and two nerites, so I moved out the small horned nerite. Left Mavis in there because she does seem to be doing some good clearing the black algae marks off the anubias leaves- I was thinking of cutting out the worst ones while I wait for new, clean growth, but it's looking better now as-is.
Funny thing- is it a coincidence, or are the snails seeking out their most similar fellows? As I thought I noticed with the tomato nerites before, now in the 38 the two horned nerites are frequently close together: Bumblebee Snail and Dimple.