Planted Tank Guru
Join Date: Sep 2006
Location: Northern California
...sorry for your loss, but that's why I recommend people to run their CO2 on a solenoid and have it turn off 5-10 minutes prior to lights-out.
Also, it doesn't make sense why your CO2 diffuser popping off will lead to MORE CO2 into your tank since the bubbles are much larger than with a diffuser. If you set the needle valve at 3bps, for example, it should stay at 3bps whether a diffuser is attached or not.
I bet you what happened was an end-of-tank dump with caused a huge release of CO2. In turn, this made your diffuser pop off, and despite the larger bubbles w/o the diffuser, the tank dump put enough CO2 into the water to kill your fish. Get a low pressure regulator with release valve to prevent this in the future.
What's the reading of the tank pressure on your regulator? Less than maybe 700 psi?