How long your CO2 tank last
I changed my 10 lb CO2 tank for my 75g about once a year, with injection rate of about 4 bps, 8 hours a day. Based on my conversion calculation, I am injecting around 10/624.666/365*1000000=43.8 ppm CO2 daily, where 75g water = 624.666 lb.
Assuming 50% off gassing loss, my net CO2 gain is probably no more than 22 ppm. I have glass top that captured some fugitive CO2 thereby reducing off gassing.
How often do you change your CO2 tank.