Thanks. The filter in the smaller tank is a one-cartridge deal, but I could just make like I do when I change its cartridges; keep the old frame for cartridge, take-off old one.. replace with new.. put old one on new frame into new tank. It will help, at least. The ridges in the bigger tank's filter are larger of course; I can wedge it in with the one that is in there now. Maybe a total die-off did not occur in the big tank? It was only sitting overnight without fish in it at my sister's place, and the gravel was damp. Ah well