Never heard of them, per se. I have heard of things like White Diamond which removes ammonia from your water. I keep some for emergency use (like a fish dies and I don't catch it) for quick removal of any ammonia that may build up. I can't say if it works or not, never had the opportunity to test it. However, I personally wouldn't rely on this type of product. It seems to me more of a band-aid than anything else. If you are doing your job as an aquariast correctly, you won't need this type of product for normal everyday use.
\Dan