there are some copper discs I think that I spotted last year... will try and find them! ours doesnt autmatically refill so we do let it run down in summer so we can clean, prob takes a couple of weeks to drink down before refilling (and we have to take the fact we are metered into consideration!)
I am not sure if it's the same thing as what the above poster has said, but there is something in the horsey mags (an advert), like a coin type thing you can put in the clean the water... Have a look when your next reading one or if your at a newsagent!