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Originally Posted by jaybyrd
In the diagram I have made examples of an air stone, a limewood block, and a track diffuser. The red indicates the CO2 bubbles, and the sizes they come out vs the sizes they are when they reach the surface. The pink arrows track the course of the bubbles. In an air stone you have large pores which create large bubbles. These bubbles both because they are large and because there is nothing to block their trip to the surface, will break at the surface in a larger form. This does 2 things: it limits the amount of time the bubble is in contact with the water, making for less CO2 dissolved into the water, and it makes for a larger bubble bursting at the surface, so there is more wasted CO2. Using a limewood block has the same situation with nothing slowing the bubbles in reaching the surface, however, the pores in the wood are much smaller than that of a stone, so the bubbles come out smaller, leaving for less wasted CO2 when they burst at the surface. When using a track diffuser, the bubble starts at the bottom and is blocked from going straight to the surface by the lengths of track. The track channels the bubbles to the surface, slowing them down on their trip, allowing for more CO2 to be dissolved in the water due to longer contact time in the water. As the CO2 dissolves along this track to the surface the bubbles decrease in size, thus leaving less waste of CO2 when bursting at the surface.
My previous comments stand, and I hope this helps to explain things better. I never implied that an air stone won't work, just that it's a terrible waste of CO2 and doesn't allow as much of it to be dissolved into the water, making it much less effective. Using an air stone would make no sense. Any money spent on investing in the track diffuser would be made up for in results and also in less wasted CO2.
I also want to clarify that CO2 doesn't "mix with the water". It dissolves into it. There is a difference.
I'm not sure I followed your reference to atmospheric oxygen. If there was atmospheric oxygen hooked up to the air stone you aren't going to be getting the CO2. The amount of CO2 in atmospheric oxygen is so small, it wouldn't serve any benefits to the plants in the tank.