The issue isn't the multi's, it's the conspecific aggression between the Julies when they mature. I tried pairing a group of J. ornatus "Gombe" in a 29 (same footprint as a 20 long) and lost all but the one that had killed off the rest. Twice. It was eventually moved to a 55 with the sole survivor of the previous group, and those two did pair up. Obviously, one of the "killers" was a female...