Well, I decided to increase my CO2 just a tad (1/8th of a turn on the big knob) and it gassed my fish. All the rummies are dead, but I still have four otos and three honeycombs. I did all the stuff I knew to do--turned off CO2, raised spray bar, did a partial water change to get the water surface down, held the fish right under the water jets, turned off the heater, etc. What else can I do? The water is pretty well aerated now (I can see the O2 bubbles).