I have had leeches in our pond for years they are all in the filters too.
I have tryed the liver and caught a few but with the size off our pond its ust a nightmare.
We have lost around 5-6 extremely large fish this year as the leaches have been on the fish then they have rubbed and then the sores got infected and died.The size of our pond dosnt help as we cant allways see the fish and only mannage to get them out when they are really ill and floppy
We have done allsorts drained the pond half down and refilled and treated all the pond and the fish.
We are also at logger heads now as to what to do next with our pond as the fish are slowly going 1 by 1.