I'm on the first fill of my 10 pound CO2 tank. It was about 1000 PSI when I got filled and it quickly dropped to 900 PSI. It stayed around 900 for about two months. Over the last week, it has dropped to 600 PSI.
What's your bubble rate? A 10# tank should last several months at least on a small to mid sized aquarium. If it's at 600 psi, it means there's no more liquid CO2 left and you're just running on pressurized gas. I'd get it refilled now, especially if you have a leak.
It depends on the regulator and what your bubble rate is set to in my opinion. 2 months could be fine with a high bubble rate. I had a 5lb with an electronic regulator that normally goes 1-1/2-2 months at 2-3 bubbles per second. It can go to 0 and doesn't dump before reaching 0. Before I upgraded to an electronic regulator I had to change my tank once the psi dropped to 3-400 psi otherwise it would dump. I found out the hard way and killed half my tank before noticing it dumped.
Check for leaks, use soapy dishwashing liquid around all connection. if no bubbling occurs you have no leaks, then it is what it is. Not all regulators are built the same. Experiment with your tank and monitor it. It was difficult for me to remember to monitor so I went with an electronic regulator. It was more expensive but it was the best investment I have spent on my equipment by far.
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when co2 is compressed, it becomes a liquid. if you hold the tank horizontally, and slowly rock it end to end, you can feel the liquid.when a tank is full, the psi could be anywhere from 800-1000 depending on outside temp. when it gets below that it is time for a refill.
i refill mine when it drops below its liquid pressure
at my house temperature its right about 800 psi
when i see the needle drop,. its fill time.. Take into account that after a refill, the tank needs about 24 hours to adjust to room temperature. you'll see the pressure change during this time
as other's have said u may or may not have a leak. my 20 lb lasts 6 weeks on my 75 gallon, and i have no leaks. just lots of waste
you could also go by weight - I've got the numbers written down at home for both my 5# and 10# tanks - empty & full weights. it's a little bit of a pain to unhook to weigh but at least you have an accurate idea of where you're at!
(this was more important to me before hosting a party to know I wouldn't run out of co2 to dispense my kegged beer... but perhaps knowing your aquarium won't go without co2 when you're on vacation??)
As said, sounds like you have a leak. My 5 lb on my 20 gallon lasted me about 3 months with a leak. My 20 lb on a 29 could last me over a year. It depends on how much you are injecting/loosing but I would check for leaks.
As for PSI, once you get it at a fairly constant temp by having it at your house, it will stay at a certain PSI, around 800, but depends on temp. It will stay at that until it's emptying out the last bit. So after it starts dropping, it's time to refill.