It would seem that I've encountered for the first time that our dorado catfish has been refusing a slightly less familiar feed (fed only occasionally) for several weeks, which led it to attack the bala shark. Of course, I cannot refute an alternative explanation involving two mere coincidences - of the switch to a slightly new feed and my dialing back the elevated water hardness, and of the return to a more familiar feed and the end of the acclimation period to the lower water hardness. But two coincidences seems too much.
