Hi bjorn,
That's a good question. To find a meaningful answer we need to think about just what it is that we are trying to do.
The goal of a diffuser is to dissolve a gas into the liquid water. If I put a teaspoon of sugar into a glass of clear water you'd agree that as long as I can still see the grains of sugar, then it is not fully dissolved. When the grains of sugar disappear it can then be considered fully dissolved.
The same goes for my gas bubbles. As long as I can see bubbles, then I know that the CO2 is not fully dissolved. Ideally therefore, I want the bubbles to disappear.
In the case of my sugar, I can use the teaspoon to stir the mixture and this will force the sugar into solution faster. If I don't use the teaspoon then I have to wait for the grains to dissolve. In the case of the gas since I have no teaspoon I have to wait for the bubbles to dissolve on their own. But there is a problem - the bubbles are on their way up and out of the tank, so what I need is to delay their escape and to keep them submerged for as long as possible, hoping that they will dissolve into the water before they can escape.
Dissolving a gas into water is only the first of two major problems associated with CO2. The second problem is now, having dissolved the gas, how do I get it to all areas of the tank to ensure contact between the CO2 saturated water and the plant leaves?
This is an issue of distribution and it can only be solved by having enough pumping power to move water quickly to all areas of the tank, especially down low, near the substrate. One also needs to manage the flow from the pump in a way that maximizes the distribution of the pumps energy over the widest possible area.
So lets look at your question again within the context of the strategy above:
Lots of tiny bubbles do nothing if they are at the surface. This is like turning you heating on and having the windows and doors in your house wide open.
Movement is key, because it addresses the second problem of distribution. If the movement keeps the bubbles submerged instead of allowing them to escape to the surface the even better because that addresses the first problem of having enough contact time with water to properly dissolve.
If you are "getting rid" of bubbles by agitating the surface where the bubbles are residing then this is a waste of CO2 and a waste of time because the precious gas you are trying to dissolve and to get to the bottom of the tank is simply evaporating.
Ultimately, you will need to devise a plan using the equipment that you have, or obtain more equipment, to fit with the strategy discussed. Try different configurations and track individual bubbles to see how much time on average the bubbles stay submerged and haw far away from the diffuser they reach before they escape. You can even put tiny pieces of paper (the size of bubbles) in the tank to see where they go in order to help you visualize the flow patterns in your tank. Do they reach the far corners of the tank? Do they travel down low?
Solving the riddle of flow and distribution in your particular tank is probably THE most important project associated with carbon enrichment. Did you follow the 10X rule for filtration/pumping? Have you considered using a spraybar? Have you considered using an external diffuser?
I can guarantee you that about 95% of the problems you will have are going to be related to CO2.
If you haven't already done so then click these links or go to the Tutorial section of the forum and read these threads:
Setting up a 'higher' tech planted tank
CO2 MEASUREMENT USING A DROP CHECKER
You can also find more information in the following threads (click the link):
Water flow in the planted aquarium?
co2 circulations?
Difference in drop checker readings in different positions
Cheers,