I am so sorry. :'( :'( The little guy probably had finrot when you got him. I am so glad you are going to replace him. Please be sure and make a quick inspection of the new one as they are prone to having fin damage and it is hard to keep them going when they are sick and have been transferred at the same time. Poor little guy.
I would get four, that way if one dies, which from stores they probably wil then you will still have 2 or 3 and if all goes well and one doesn't die then you'l have four and they'l be a lot more active and fun to watch, I miss my three that died. My lonely one now is not afraid of anything actually, strange, he'l come out and go by the angels in full day. That is if your not overstocked.
The little fish is not bred in captivity but caught in the wild but since they are so quick moving, they are subdued by spraying the water with cyanide and catching them in nets. This damages their internal organs and costs many of them their lives, but since they are being caught out of rivers and streams the ones harvesting them have no cost in them so they do not care if there is high mortality for the profit margin on the fish that do live is 100%. Then when they do get to the local pet store a lot of the time they are fed flakes a couple times a day. Otos are grazers and need to have food available all the time. Since they eat brown algae, and the tanks at the pet shops are scraped and kept pretty much algae free they are pretty much starved. If they do not sell right away many of them do die of starvation. When they have gone hungry past a certain point of time the bacteria that resides in their stomachs that digests their food dies and them even if they are sold and fed well the food actually poisons them for it cannot be digested and used by them. This is the reason that you should never buy an Oto that does not have a pudgy little tummy. You also want to make sure they do not have bright pink areas (hot pink) on their tummies as that is a sign of internal bleeding or infection.
When you bring them home put them in the quarantine tank for the first 2 weeks and give them food at night as they are nocturnal and will eat most of their food at night, but do not remove the food that is uneaten in the morning because they will eat it. The algae wafers will not foul the tank in the amount of time you will have them there.
Until the fish industry starts to breed this little fish in captivity they will more than likely have a higher than normal mortality rate, but they are worth it and I would not (for one) have missed the experience of owning these wonderful little fish for anything.