How Much CO2 in a 75g?

[Saltinthedesert]

I recently added a CO2 setup to my 75g “moderately planted” tank. I’m not totally new to CO2 setups and have run one on a 20g for a year…

but I’m wondering what kind of bubble count I should be looking for on a larger tank? How many bubbles in 5 sec is a good starting point for a 75g tank?

[Mmiller2001]

Bubble count is irrelevant. You want as close to 30ppm CO2 as possible which is a 1 to 1.5 pH dropped from a degassed sample of your aquarium.

[PlaneFishGuy]

@Saltinthedesert I'm running about 5-7 bubbles per second in my 75g . This keeps my drop checker are the slightly darker side of green so its fairly conservative. I am leaving a margin of error for the fish. I also have a lot of flow from my HOB and an slow airstone so I am assuming there is some waste. Your set up may be more efficient. 

The challenge I also had in the four foot tank was even distribution, so I had to work on improving flow throughout the tank. 

Edited September 28, 2023 by PlaneFishGuy

[Saltinthedesert]

@Mmiller2001 Explain this like I’m 5. 
 

“a 1 to 1.5 pH dropped from a degassed sample of your aquarium.”

[Mmiller2001]

On 9/27/2023 at 1:29 PM, Saltinthedesert said:

@Mmiller2001 Explain this like I’m 5. 
 

“a 1 to 1.5 pH dropped from a degassed sample of your aquarium.”

Note that BPS means very little and is nothing more than a crude way of measure injection rate. We want to actually measure the amount of CO2 in the water and the rate injected varies between ever persons tank. How to measure CO2 then? Either using a pH/KH chart, which is also not accurate, do to other factors I won't mention here, or to measure the pH drop from a CO2 degassed state to actually CO2 in the water. We know that 30ppm CO2 is near the maximum amount we can inject and beyond 30ppm we can stress or even kill our fish. 

The idea is then to determine the amount of pH drop from atmospheric equilibrium to an injection rate that reaches at or near 30ppm of CO2. This all has been well documented and is stated to be a 1 to 1.5 pH drop. Take a sample of your tank water. Enough to pH, either by a liquid test kit or pH meter. Let this sample sit exposed to the air for 24 to 48 hours ( there are other methods but won't mention them here). Ph this sample. This is now your degassed ph and is the baseline pH. Once CO2 injection begins, meaasure pH. You are looking for a 1 to 1.5 pH reduction from your baseline degassed pH. if you are between a 1 to 1.5 pH drop, you will know you have enough CO2 in the tank (about 30ppm of CO2).