I copied and pasted this out of a thread from a few weeks back:
"Otos are pretty much 100% wild caught -- and as all invertebrates, they cannot directly digest cellulose and plant fibers. There is a colony of bacteria in their gut that does this for them. (Just like a cow's stomach for a very good analogy.) Since they are wild caught, the trip can be long, and if the otos do not get a new source of food, their digestive systems begin to consume the bacteria in their gut for sustinence. The problem is that even when food becomes abundant again, there may not be enough bacteria remaining to grow fast enough to support the fish. So, even with plenty of aglae and cucumber and other food available, the fish can starve top death because they have no way to change the food into something digestable and usable by thier body. So, lack of food combined with another environment change and all the stress therein can often be too much for the poor little guys.
Now, the reason you are advised to only buy the fish that have been at the LFS for a fair amount of time is that you can be reasonably sure that that fish has been eating and it bacterial colonly has not been scavanged. There is an external sign, as well -- a well-fed Oto will have a pudgy little tummy, almost like a Buddha. Conversely, one that has had to scavange its gut bacteria will have a little bit caved in tummy. So, next time, look for the ones that have pudgy little tummys -- most likely the ones that have survived the transit the best. "