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Re: pros and cons to DIY c02 injectors.
some people use bubble counters and try to slow the bubble rate witha controll valve or clip to like 1 or 2 bubbles per second. with a diy bottle the first day it starts kicking in, bubbles will be way faster than that. you will notice a ph change at night the co2 will drop your ph a bit and should go back close to normal during the Day.
here as a friend put it to me in a Email once.
Running an airstone will help keep CO2 levels the same as what is in the air around the tank (or rather, the pump). Bubbles and surface agitation create more surface area for gas exchange, which keeps the dissolved gas at the same concentrations as the surrounding air. This works well for low-light plants and somewhat well for mid-light plants, but usually doesn't provide enough CO2 for rapid growth of mid-light or high-light plants.
This is why surface agitation is actually bad if you have CO2 injection, as the gas exchange tries to even things out, dropping the CO2 to normal atmospheric levels.
I disagree that pH shift due to CO2 is inconsequential. pH shift is pH shift, and can be harmful to fish. kH shift is a matter of hardness, not pH, and is a different thing entirely (though it does limit the pH, generally). Many aquarium fish will tolerate such a shift, but some are extremely sensitive to it, and will not do well with a daily rollercoaster like that. If using injected CO2, it is a good idea to know the fish you are working with and whether or not they are tolerant of changes in pH.