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Inline diffusers are great and fairly easy to build. However if you just want a ceramic diffuser, the GLA atomizers are my favorite. They create the finest mist I've ever seen a diffuser make and allow you to use less CO2 and still achieve optimum levels of gas in your water.
Folks might laugh, but I've been using a couple of 'cheap' atomic CO2 diffusers from that auction site that I paid $12 a piece for and they're still going strong after 4 years. Can't see paying any more than in all honesty ! They are also on smaller 10/20 Gallon tanks. Any larger I'd go the 'Griggs' style as advised above.
I used a cerges reactor on my 60g, to be honest I liked it a lot more than the Griggs I’m currently running on a smaller tank. Just easier to use and maintain.
I purchased the Sera large,Max mix large and UP inline diffuser with a porous core . I have a fluval 406 the UP seemed the most logical small convenient no moving parts. I noticed that it can dissolve up to 1000 CO2 microbubbles in a minute, which is plenty for a 100-gallon tank. On the other hand, if you have a small aquarium or do not have that many plants, you can dial the pressure down.