I tried a chopstick diffuser and cotton balls. I tried several times and different ways, but I just couldn't get it to work great. Some small/micro bubbles came out, but some bigger ones came out too. My best success was stuffing the chopstick further up the tube, and then stuffing cotton in after it.
I wasn't happy with it, so next I tried a cheap ebay glass/ceramic one just like THIS ONE
. It worked better, but still had some bigger bubbles. I ran it for about a month, but I didn't like the bigger bubbles, so I figured I'd try one more diffuser.
Now I'm using a Fluval Ceramic 88g diffuser
. It's bubbles are all very small and more consistent, and it's much better than the others.
However, the Fluval isn't perfect for DIY CO2 either as it does require far more pressure. My DIY CO2 bottles are much harder and more pressurized than when I used the other diffusers. I think it's fine, but it's an observation. It also takes several hours longer to build up pressure and get running after you depressurize the system. Now I'm using 2 bottles with check valves between them, so I can swap them out 1 at a time and not loose pressure.
Also, the fluval is by far the smallest, and since it's black it hides the best also. It's the least obvious in my tank, and now I wish I had black CO2 line as well.
Here is the chopstick:
Here is the cheap glass one:
Here is the Fluval one: