In article ,
says...
That would depend on the type of algae and whether the tank is planted or
not. If the Angel is not too big, you might try 3 or 4 Otos.

I've got 3 otos and I'd have to agree with those that say they get lazy.
And I only feed them once a week. The one SAE in the same tank does ten
times as much algae cleaning as the 3 otos.

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BNSF = Build Now, Seep Forever