PPM - 101
Think about a 1 liter bottle of air and compare the mass to a bottle filled with water. There are many more "parts" in the water bottle and more "millions of parts" because of the waters higher density so you cannot compare part per million directly with a bottle of air.
The amount of one fluid (the solute) that dissolves in another (the solvent) depends on the pressure of that solute within the solvent. If I cover the bottle of air I will only be able to dissolve more CO2 in that air if I increase the pressure of the CO2 and the pressure of the air. If I then uncover the bottle both the pressurized air and CO2 will escape until the pressure of the air/CO2 mixture is the same as that of the atmospheric pressure. If I cover the bottle and climb a mountain and then uncover the bottle, then immediately more air and CO2 will escape from the bottle until the pressure in the bottle is equal to the new (lower) atmospheric pressure. This is called (unsurprisingly) the equilibrium pressure. No movement of CO2 or any other gas can move from atmosphere to the bottle unless either the pressure in the bottle is reduced or the pressure in the atmosphere is increase.
So the amount of molecules moving from atmosphere to water is based on the barometric pressure. If the barometric pressure rises then CO2 moves across the waters surface into the tank and if the barometric pressure decreases then CO2 moves out of the water because it's pressure is higher than that of the atmosphere. Since there are many more molecules of water than there are molecules of air the calculation of ppm will have a different value. 400ppm of CO2 in air cannot have the same value in ppm in water directly. The calculation "ppm" is really number of CO2 molecules divided by the number of water molecules or, number of CO2 molecules divided by number of air molecules.
The value 1 part per million in water means that for every single molecule of CO2 in the bottle there are 1,000,000 molecules of water. So, for a given rise in CO2 pressure, a certain number of CO2 molecules will enter the water but since there are many more water molecules than air molecules then the distribution of CO2 molecules is "thinner" in water than they would otherwise be in air.
This is why CO2 is injected into the tank under pressure. So if your needle valve reads higher than zero this allows more CO2 to dissolve in the water than can dissolve from the atmosphere alone. The higher the needle valve reading the higher the CO2 pressure and the more gas dissolves. However, since the dissolved gas is at a higher pressure than the CO2 in the atmosphere the CO2 moves across the surface into the atmosphere attempting to reach equilibrium (or, the same pressure) with that of the atmosphere.
Sending compressed air through a diffuser does nothing beneficial, firstly because the mass ratio of CO2 is very low (400 molecules of CO2 per 1,000,000 molecules of air) and secondly because the disturbance created by the bubbles breaking the surface drives what CO2 is available out of solution in exactly the same way as the shaking of a fizzzy drink bottle drives off the CO2 and makes the drink go flat.
In a non-CO2 environment, as the plants absorbs CO2, it's pressure within the water drops and more CO2 in the atmosphere which is now at a higher pressure moves across and into the water to replace what was depleted - unless there are bubbles or surface disturbance which drive off what is held by the water. There is a race between what is absorbed and what is depleted by agitation. Agitation normally wins.
Liquid carbon products do not have the same effectiveness or efficiency as CO2 gas because they are not 100% CO2. There are a complex aldehyde which must be metabolized and converted to CO2. Therefore the CO2 yield at the end of the conversion cycle is no where near that of 100% CO2.
Hope this makes sense...
Cheers,