A trio is minimum. Otos are a schooling fish.
Here is some info vital if you are going to keep otos;
Otos can only digest food if they have a certain bacteria in their stomach. Without this bacteria, the oto will starve, even if it has eaten lots of food, because it cannot digest properly.
These bacteria need food, and will starve and die if food is not available.
Lots of otos don't get fed during transportation, and even though they are alive, they don't have any bacteria in their digestive system and are doomed to die.
Healthy otos with good bacteria will look fat in the middle, like a buddha belly.
For this reason, don't get Otos from a new shipment, wait a couple weeks and pick out the healthy ones.
Also once you have otos, make sure you are feeding them with zucchini, cucumber, algae tabs and occasional protein supplements. Otos have been known to starve once all the algae in the tank is gone.
