I just finished completing two black water tank set ups with my new RODI system. One houses a pair of cockatoo and ten n. beckfordi and the other another pair of cockatoo of a different morph and ten h. erythrozonus. Here is the first tank.
Just an fyi: n beckfordi is one of the few species of nannostomus with a reputation of hunting frys. Doesn't mean they will get all of them but they are likely to get a few.