>You might try a denture cleaner to do the job. In the US "Alka-seltzer" is >used by some to clean air stones, ceramic diffusers and other hard-to-clean >aquarium devices. There should be a locally produced product for dentures >in the Czech republic that you can try. Woops... I meant "Efferdent" not "Alka-seltzer"! :-) Aquarium Plants & Driftwood http://www.floridadriftwood.com