Otos Disappeared!?

If the tank is heavily planted and has lots of hiding places, I would wait and see. Though if you are concerned one or more might have died I would look for them as dead, decomposing fish can cause ammonia spikes.

Ottos need to be constantly grazing algae and five in a 40litre will need a lot of algae.

How new are they? Ottos have a very tough time of it from their collection from the rivers by cyanide, transport without enough food and not enough algae in the dealer's tanks. If yours are reasonably recent they may just be too weak to survive, I'm afraid.

I've got 5 ottos in my tank and they've worried me quite a few times (worry is a slight understaement) and if you've got a heavily planted tank like mine they are very hard to find, I go days only seeing 1 or 2 but another day I'll look in and see them all in one spot.

edited to add...they're also very good at attaching themselves to the underside of logs in upside down mode and I've noticed mine also hang vertically from plant stems at the back on the tank making it very hard to find them.

ps: try some blanched cucumber or courgette as a treat, they love it

Yup...I'll second that! The times I have thought that my little otos must surely have died and been eaten, couldn't be counted, and then I've spent ages looking for their poor little bodies....just to spot them two or more days later zipping to their latest favourite munching place.....little buggers