The issue with Otos is the way they are caught and shipped, and the way they digest food. They use cyanide to stun them, then don't feed them while they are held for shipping, and they starve.
Otos are a bit like cows. They rely on gut bacteria to digest the cellulose in algae cell walls, to get the nutrients inside. The bacteria provide nutrients with their digestion too. Once caught and unfed, the bacteria begin to die off. Once they get to a store, they are faced with unfamiliar food as well as being starved. Not all will take the algae tabs, which are nothing like what they are used to.
The Germans call what they eat Aufwuchs. It means the algae plus the infusoria, bacteria and tiny animals that live in the algae. I have seen Otos eat microworm, but I think it was only because the worms were on top of algae. In habitat they graze constantly. In a tank, they will eat biofilm, which grows really well on wood, btw, but it is not algae and it is not enough for many of them.
Those that are weakest die within days of reaching a store. The toughest of them last longer but until the gut bacteria populations regrow, they are not getting much value from what they might eat. They can look fat as can be, but still be starving.
It is best to buy them only after they have been in a store for at least one week, preferably much longer than that. Then they really need algae to eat. I keep marble chips in a jar full of tank water, left in the sun or sunny window, 'til it gets green and fuzzy. I drop them in the tank, they are snow white again in a day. Back in the jar. Takes a LOT of rocks to keep a few Otos fed, and the snails and shrimp love it too, so they compete for it. I try to keep enough on hand for all. Most of them will take algae tabs or pellets eventually, but perhaps not in time to get their guts working again.
If they survive two months in your tank, they likely will live for many years. If you can, get to the store when it is feeding time, buy the ones that you see eating the algae tabs, and that are most active. Those just sitting on the tank bottom, leave them, poor things.
I really hate they are caught as they are, but it is not likely to change so long as they are being purchased. They are charming and very hard workers, and I love them, but it is not that they are so sensitive. It is that they are starving, and weak, which makes them ultra susceptible to any opportunistic infection around until they get stronger, and often it is simple starvation that is the cause of death.
