I used to keep freshwater fish now I have a marine tank with a lot of invertebrates among fish, corrals, starfish, worms anemones etc. Invertebrates are delicate organisms.
I have not had any experience with leaches but changing the water chemistry may help try making the water more acidic or alkaline, you could also try some stuff that you may find at your local fish shop that helps you get rid of snails.
I read that a freshwater pufferfish may eat leaches, you can try one of them.
Invertebrates are also very susceptible to copper, you could try this, in a marine tank you have to be very careful of copper leaching into your tank which can have disastrous results on invertebrates. I believe that most snail killing pesticides employ copper in them to do the job.
You could try wolf's method, but if your leaches are not hungry it will not work
Hope this helps,
I think I figured out a solution, I removed all the plants from water, took out all the drift wood, boiled the wood and did some salt water treatment. I then boiled the sand too.
Then I filled in freshwater and added a full packet of salt (my aquarium holds 1400 liters of water). I saw the remaining leeches die and I dont see them anymore but Im not sure as I plan to remove some water this weekend and put in some fresh water.
I have another aquarium which holds 500 litres I plan to convert the same into Marine tank. It is difficult to maintain the Saltwater aquarium and 1400 liters will be a lot.
The local aquariums dont have any suggestions on leeches, they are far too pleased to sell you the fish