They'll lower it if they're pulling in high co2 air from the surroundings (learned in Randy’s Chem fm) and they’ll raise pH if the ambient air has less co2 than the tank
That answer is waste gas dependent
We run our pico reefs solely on air so it's ok for corals, but we use special lid fits that prevent salt creep from happening. Normal tanks get lots of creep so airstones are less common in display agreed
Aquatic eco detailed surface boil and I never forgot it:
Stilled water is flat and has X surface area where gas exchange is always taking place even if we move nothing
Moderated by variables such as fats/oils and various ambient conditions, the only way to increase o2 and co2 out given all other conditions was to create surface boil, the ripple effects on the water whether by powerhead or airstone. Now the flat plane is a series of little mountains, massive surface area increase for the gas exchange already set by partial pressures and various water conditions
So it’s not that we are increasing rate of exchange, as much as we are increasing surface area for an exchange already in place
The surface boil from an air stone typically beats that of a powerhead, though they have negatives that curtailed their use in displays in modern times in aquariums. in lake management and river management, they use air not water pumps
Perhaps a powerhead one inch under the surface shooting up a three foot fountain would win, but in practical tank settings given controls of the negative characters, an air stone will beat even an oversized powerhead for oxygenation. In emergencies this is helpful
On a battery setup the cheap bait box bubblers will run days or weeks, a powerhead only a fraction and the tiny bubbler will be the most egress and ingress one could get.
The aquatic eco education points always included the aspect of laminar flow vs random flow when dealing with oxygenation in systems management: laminar flow straight up and down cyclic is the pattern of the air stone, a round current pattern which is the most refreshment you can get for surface-mediated exchange
Anything other removes efficiency, like when a filter system has its inlet too close to the outlet and double filtration occurs, laminar flow directly promulgates air exchange more than random flow + inherent surface boil= air stone unbeatable but only for exchange...which may or may not be needed. In lakes experiencing fish kills merely making a thirty foot tall column of bubbles can stop the kill and change the limnology of the entire lake within five months. That site was sick, it’s a sad loss. They were eco system masters agreed
