Arrrgh flatworms

[essencereefer]

So I have read a bunch on these little bastards. I have two stags that have been attacked. I can get my hands on Lavamisol for freshwater. I did not see whether this was acceptable. I have digital scale so no worries on measuring. Just want to know of anyone else has used the freshwater (a) and (b) how much did they use.
This will be a display tank nuking unfortunately. My colonies are far too big to take out.
thanks in advance.