Do you have a picture or at least give a description.
If the leeches killed your fish, trust me you would have noticed them. Odds are these guys are just looking to help you with tank maintenance. Fish tend to decompose very quickly and the cleanup crews are very efficient.
Not meaning any offence, but leeches generally require poor water conditions and they will hang out until the water conditions improve.
You have a good start large water changes, 50% plus every 3-days for a couple of weeks, clean every hard surface as well as you can, remove any dead organic material. You can try removing the leeches themselves, but until the water conditions drastically improve they well come back fast.
While you are in there make sure there are no stagnant area, water movement is our friend.
Generally a minor problem in the scheme of things.
Though it is always a good idea to clean, sanitize and/or sterilize (obviously living things cannot be sterilized), the fact is you do not know where the leeches came from, these things simply exist and when the conditions dictate, they appear. It’s a mystery!