As mentioned above, boiling is an effective method for cleaning an air stone. A soak in bleach, hydrogen peroxide, or vinegar followed by a good rinsing are other methods that can be used alone or along with boiling. I do not have air stones anymore but I do have ceramic diffusers for CO2
diffusion and use any of the above methods for periodic cleaning.
Just keep an eye on boiling plastic parts; they can melt and/or warp if boiled too long.
Off topic: Do you still have the clown loaches?
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