While the outbreak of 'Brand Spot Desease'( Brandfleckenkrankheit) it's too late for sure and the crabs,which are taken ill, will die after a few weeks.
Some red to brown spots will appaer first on the carapace of the ill crab.When getting more and more the colour becomes darker.
Responsible for this desease are some 'thread-fungus'.But the real cause will be anger and stress in different ways for a long time. Just like too many crabs in a small tank. Permanently territory fights will follow and out of this maybe competition for food. Requirements like this are contributing different ways of immune deficiencies and deseases.Also bad waterconditions ( Nitrat) and different temperatures inside the tank are able to stress your crabs and decreases their immune system.And this again is finally beneficial to some fungus species making the crabs sick or worst-case kill them !
The offer of beech and/ or alder leaves to the crabs are maybe able to dam up infectious deseases. But it will sure prevent to them, when fed regularly to the crabs. In my tank beech leaves are preferred.
When the 'brand spot desease' is coming to your crabs, you must act very careful. Some few pieces( german: 'Sporen') are able to exterminate your whole crab population at all !!
After removing the dead crab bodies ( best is to burn them out), you had to clean and desinfect the whole tank and things that are used.
A medical treatment with preparation is not to be recommended,because these medicine often contains some copper.
And unfortunately all crustaceas( and all snails too) are too sensitive to copper and will die soon with small amounts of copper.
Finally you have to realize,mostly these medicines don't really help.