My classroom aquarium has some albino corydoras catfish in it. They are filling the role of “bottom fish”. (The tetras swim in the middle, and the guppies swim at the top.)
Much of the time, the corys are swimming around the tank, scouring the gravel for anything they might eat. When they want to rest, they go to the part of the aquarium where the water is still. It’s also a place where some java moss is growing. The fish like to rest in it.
When the corys rest, they’re very still. It’s starting to happen less frequently, but for a while, it seemed that every other day a student would urgently call me over to the aquarium to see the “dead” fish. Then it would move and they’d tell me to never mind.