How does your regulator high pressure gauge read

Sits at around 800 PSI until it starts to drop. (5lb tank). Takes a few days to drop to zero in my experience though.

The high pressure gauge on a 2 stage reg should drop very quickly as the residual CO2 will no longer be in a liquid state

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It is likely to be more gradual at cold temperatures, as in the drop will be observably more so, in my experience. Before I got into this hobby my co2 usage was in homebrewing and I had my tank in the fridge. I could notice the drop and have ample time (days) before I would take it to refill. As my aquarium tanks are sitting at room temperature once I see that needle move I usually take it right away to avoid going without co2. Again, this is probably subjective though, since I only pushed beer at around 10-15psi and in the aquariums I have it usually set to around 25-30 psi which would give me a less gradual drop.