A saltwater bath can get them to fall off. about 4oz of salt per gallon disolved in warm water and poured into a bucket of tank water. Put the fish in for about 15 mins, It should get them to release. It'll clean the fish up pretty good. Some types of leeches are more salt tolerant than others but it should do the job. Just watch the fish for any signs of distress and remove to fresh water if it goes belly up. Nasty things.
