I had this problem of my air stones getting clogged, and I can not buy air stones here so it has been a big problem for me.
I tried and asked...and researched...nothing really worked fast and easy...until I took out all my household cleaners...and tried them one by one...
The best was spirit of salt. (hydrochloric acid)
It just takes 3-4 second to dip the stone in it while it is discharging air and it is like new.
U dont even need to wash it after that...just drop it in the pond...Hydrochloric acid is not poisonous , it is corrosive if undiluted...but not poisonous. ( u have it in your stomach)
So I hope this will help U clean all your clogged air stones...

Neli....I have heard that vinegar is the best for cleaning airstones.....worth a try I reckon

Clif, it is soo good!
Just dip your stone inside 3-4 times...roughly 3-4 seconds...and it is done.
U can buy it from any supermarket...
I keep a jar near the pond all the time.