Many fish are quite good parents but you cannot expect a tiny fish to defend a territory against something several times their size. I have seen rainbow cichlids, with a size of only about 3 inches, defend their fry against a tank full of fish up to 5 inches in length, but they would fail against a tank full of adult oscars. Everything has its limits but that does not detract from the fish's attempt to protect the fry. Sometimes they simply have no chance to succeed due to size differences. The B. imbellis, at only 1.5 inches adult size, are going to have a hard time in almost any community tank.