The bamboo shrimp and everyone else are fine though.
What gives? If the shrimp and the snails are ok, why would the otto die?
There is still a ton of algea on the sides of the tank so it can't be to lack of food.
The octos are a rather frail fish, very easily excited. do not blame yourself you have no way of knowing what the fish went thru prior to your ownership. everything dies, I have lost some very good friends and family. why did they die and not me? cheer up! death is a part of living. octos are not exclusive algae eaters, as with cories etc try to get some flake etc to them at least once a week. was it being pestered by some other animal in the tank?
One of the biggest problems of trying to keeping Otocinclus catfish is during the first month of ownership. After that, if they are still alive they should remain so unless major mistakes are made by the fishkeeper. Otocinclus start out at the fish store in bad shape.
So, it's probably not your fault.
If only all aquarium livestock arrived in the condition that Mustafa's livestock arrives. The world would be a better place.