I use a check valve on my DIY CO2, and right before turning off the lights at night, i unplug the tubing and let the CO2 diffuse into the air at night, not in the tank. I also run an airstone at night, even when the CO2 is not running. Then I attach the tubing back to the check valve in the morning, and it takes about 10 minutes for the bubbles to begin to show beneath my intake tube. Just my experience. I've had no fish problems since I started it.