Dear All,
This is an interesting subject.
There are a few solutions (pardon the pun).
Copper Sulphate is not as silly as it sounds. Malachite Green is use to 'sterilise' fishponds of fish fungus growth, and I have used it very effectively... at far higher doses than recommended out of desperation, and it has caused no problem. If there are no fish present, copper is not a bad idea, and to stop people pinching it, just cut it small and throw it about... far too much trouble for them to go fishing for it! Smaller it is, the more the more worthless the task in retrieving it... and the more likelyhood they will get caught!
Next. Whether fish present or not, salt could work. In the past, I have used salt to cure my own and others Coy of fungus... a kill or cure situation, but I have proved it to work, and am not afraid of using it. Raising the salinity % to quite high levels acts as an antiseptic, and, surprisingly does not cause distress to fresh water fish if done in a controlled way. Again, I have found this out in practice and out of desperation for a cure, and it has worked very effectively.
Can suggest more if wanted?
Regards, Bernard