Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
The pressure in the CO2 tank remains fairly steady until it's almost empty. Then it starts dropping relatively quickly. The more the pressure drops on the input side of a single-stage regulator, the more the pressure rises on the output side. And that means more CO2 flow through the needle valve, so the tank empties faster, which makes the situation increasingly worse.
For example, if I set my single-stage regulator to 15psi, and allow the CO2 tank to empty, the output will rise to a maximum of 30psi.
If you have a single-stage regulator, a small CO2 tank that completes the final stages of emptying quickly, and an efficient diffuser than can successfully dissolve all that extra CO2, it can result in disaster.
Many folks thing tank dump is a myth, having had a tank empty without disaster. But more likely they've never seen it because they're lacking one or more of these conditions. For example, my fish survived one dump because the reactor was already at the diffusion limit and the excess simply burped out, with little change in ppm. The next time, I had an Atomic diffuser with no such limit, but it was also survived because I was using a 20lb. CO2 tank. Since the increase occurred gradually over two weeks, the fish successfully adapted, even to the final value of 60ppm. If that had been a 5lb. tank or smaller, the levels would have risen faster, and my fish would probably have been killed.