They are very difficult to eradicate. You can dip the fish in a salt solution to loosen them then remove them with tweezers. To remove them from your tanks, it will be a lengthy process. You will need to treat with an organophosphate insecticide. Cyropro is a newer formulation that uses a less toxic insecticide but I am not positive if its effective on leeches. Older DTHP formulations such as Clout will also work but are more toxic. Dimilin or malathion are other options but are also quite toxic. The insecticides will kill the adults only, not the eggs. Almost nothing kills the eggs, including bleach, heat, cold or drying. So, treatment needs to be lengthy (month, usually much longer) to kill new leeches as they hatch. Leeches are hermaphrodites and only a single leech is necessary to start an entire population. Not fun as I have dealt with them before.