What do Otos eat exactly?

my tank is at 77 degrees. Sadly, one of my otos died today :( hopefully the other 2 will be ok. I have no idea whats wrong. Ammonia is 0, Nitrites 0, pH 7.8 water temp. is 77.
 
The otos in my gf tank are very active during the day. They'll go around sucking on the sides of the tank, the plants, driftwood, gravel and anything else that is in there. I dropped a piece of cucumber in there yesterday, but they seemed to ignore it for the most part.
 
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