If you've only had them a month then that could partly explain the deaths, oto's are notorious for being difficult to acclimatise successfully to tanks- the reason for this is because many are half starved by the time they end up in the petshop. Put the stress of a lack of food on top of being made to acclimatise to a completely new tank set up after being bought, many oto's don't make it.
Another reason is because they have a beneficial bacteria in their gut which helps them digest their food and get nutrients out of it, some people say that when the oto's starve during transport they start to lose this beneficial bacteria, which can make it even more difficult for them to gain weight again after being bought. On the plus side though, if you buy a large group of oto's they will help transfer the good bacteria to each other, and help each other regain their strength (they are also sociable fish and do best in large groups).
My guess would be that the oto's death was down to either;a
a. Lack of nutrition in its diet from being starved in transport to the petshop and then not acclimatising well to the new tank.
b. Internal bacterial infection, which can cause sudden deaths in fish.
My advice would be to only feed the oto's algae wafers and pellets after the tank lights have been turned off so they have more of a chance of eating them, and to buy more oto's (another 2-3 would be good) and to treat the tank with an anti internal bacterial med (like "anti internal bacteria" by interpet) to be on the safe side
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If you have angels in the 10gal, they will certainly outgrow it as time goes by, do you have a larger tank lined up for them when they outgrow the current one in the near future? How large are the angels at the moment and how many of them do you have? Depending on their size, i would advise planning to upgrade their tank very soon.
