Wooden air stone.

[mansiz]

My wooden air stone for the small protein skimmer seems to turn black, and the bubbles it produce (the air pump is on with the same power) is getting lesser, does this means I have to replace it?

[Biggen]

I would say that if the bubbles it is producing are getting less and less, I would buy another one. Relative to the cost of other equipment in this hobby, those are pretty cheap to replace.

[captivereefs]

You should replace them about once a month.

[rayjay]

I found the best and most economical way to use wooden air stones was to have a few of them on hand and change them BEFORE the get real bad.
After removal from use, clean the stone and dry it out completely, and you will be able to use it again, almost like new.