A CO2 diffuser is a device you push CO2 gas through that breaks the gas into microfine bubbles so the CO2 can more easily dissolve into the water (forming carbonic acid). I use a ceramic/acrylic/glass one from AquaRio and it does a nice job. They bill it as a 2-in-1 diffuser and bubble counter but I’ve never really paid much attention to the bubble counter aspect. The ceramic pad is porous and forcing the CO2 through the tiny holes in the pad under pressure is what breaks the gas into microfine bubbles. Smaller bubbles have a larger ratio of surface area to volume than larger bubbles (the ratio scales as 1.5 / D where D is the diameter of the bubble) which gives the CO2 gas more physical exposure to the water. The smaller bubbles also last longer than larger bubbles: larger bubbles shoot straight up to the water surface and break while the smaller bubbles swirl around in the water for a while before eventually making it to the surface. This additional time also promotes better diffusion of the gas into the water.
Cleaning the diffuser
The diffuser works great until it get scunged over with algae whereupon the flow of CO2 through the diffuser is greatly reduced or blocked altogether. You can temporarily overcome this by increasing the pressure of the CO2 but this isn’t sustainable and my preference it to clean the diffuser rather than mess with anything upstream. Removing the diffuser from the tank and using a syringe with a small piece of tubing to force straight bleach (‘thin’ bleach, not ‘thick bleach’) through the ceramic diffuser does a wonderful job – just be sure to thoroughly rinse afterwards. I have also tried hydrogen peroxide but in my hands that doesn’t really work. Lately I tried just temporarily removing the CO2 diffuser system from the tank, and without dismantling it at all, layering on a couple drops of straight liquid carbon. To my pleasant surprise that also seems to work well and means you don’t have to take the system apart (which is a hassle) or worry about getting it all completely rinsed afterwards – just reinsert the drop checker system with the liquid carbon still on it back into the water. Maybe if the diffuser is put in a shaded part of the tank, e.g. because it is underneath plants, having it in the dark will also inhibit the growth of algae on it.
Do you really need a diffuser?
I suspect and have done some supportive empirical testing to show that if you have a closed lid tank (as I do) you do not need a diffuser. The rationale is that because CO2 is more dense than air, in a closed lid system the CO2 bubbles whether microfine or not get to the surface and make a blanket of enriched concentration CO2 gas over the entire surface of the water, representing a huge surface area for CO2 diffusion. This won’t work with an open-lid tank since vagaries of air currents in the room, including potentially convection caused by the heat of whatever lighting system you have over the tank, will make the CO2 diffuse into the room much faster than it can diffuse into the water. For a while before the diffuser arrived, I connected up the CO2 source to the main air line bubble system such that instead of bubbling air through the central bubble tube, I was bubbling a CO2/air mixture. The drop checker said this was working. So why not just do that? I feel I have better control over the CO2 with the diffuser, in part because forcing the CO2 through the diffuser makes the diffuser act as a mini-regulator of the CO2 pressure, and in my small tank keeping the CO2 pressure both low and controlled is challenging. That and because it’s fun to watch the CO2 microbubbles swirling around in the current…