There are so many diffusers on the market these days it's probably a hard decision as to which one you should use. The problem is there's not a great deal between them so this makes your decision even harder. So I've decided to focus on just one - it looks great and functions well too and it's called the AE Design 150L Spiral Glass CO2 Diffuser.
What's special about this product?
Delightfully manufactured it looks super in the flesh. The ceramic plate is about 2cm wide and made of sufficient quality to allow for small CO2 bubbles to pass through and then circulate around the aquarium. The overall height is about 10cm. What really works well for this product is the spiral which always captures hobbyists eyes. The CO2 bubbles pass from your regulator into the diffuser and then pass up through the glass spiral and this in itself becomes mesmerizing. It's hard to take your eyes off it...
As you can see, because of the spiral, it doubles up as a bubble counter which means you need one less piece of equipment. This glass diffuser is suitable for aquariums up to 150L but if your tank was a little larger, I don't think it would cause a problem. Similarly if you have a smaller aquarium and are particularly fond of the spiral (it is nice isn't it) then you could still use this diffuser. Remember the lower the pressure on your regulator, the less bubbles come out of a ceramic plate. The higher the pressure and bubbles come out of them everywhere.
Keeping diffusers clean:
Cleaning glass diffusers is easy - algae builds up on the plate every couple of weeks so to keep them fresh, soak them in a neat bleach solution over night, and then in the morning give them a thorough rinse under tap water for about a minute making sure it's nice and clean (and bleach free).