Should I use water or mineral oil to fill the bubble counter of the Aquarium Co-Op CO2 regulator?
Regular tap water can be used to fill the bubble counter of the Aquarium Co-Op CO2 regulator so that you can see the approximate rate at which CO2 is entering the aquarium. However, the water will evaporate over time, so mineral oil can be used instead so you never have to refill the bubble counter.
For more information on CO2 systems in high tech planted tanks, read the full article.