Hey Nikolay,
The reason I questioned the solenoid is that on the APD I recall only about four "CO2" poisonings. The first one I was able to figure out exactly what had happened. The person had a tight fitting hood on the tank. We know that oxygen is absorbed into the water at the surface. Put plastic wrap on the surface and the fish will suffocate. Also, oxygen will not be displaced in water by CO2.
Well the CO2 was slowly escaping from the water and into the trapped air above the water surface. This was pushing the oxygenated air out of the trapped air space. Pretty soon this air space was oxygen deficient. The fish actually died from suffocation from lack of oxygen, since none was being absorbed at the water surface.
Three of the four occurrences had tight fitting lids. The last person was an expert, and he said it was CO2 poisoning, so assumed he knew what was happening.
I happen to have open top tanks and high KH water. I just have the feeling that my pH will never get that low to cause higher CO2 and kill the fish. Thats why I dose the CO2 24/7, without a solenoid. I don't know about KH=2 water. That may be dangerous. I would always aim for a KH=5 or greater.
Also if you're using an in-line reactor the gas is only going to collect at the top of it. Since the power is out the pump won't be pushing CO2 enriched water into the tank. If you have a diffuser, the action of the CO2 bubbles will cause circulation in the water.
Regards,
Steve Pituch