Next time, soak your turtle in regular tap water. Deep enough that his shell is completely covered but shallow enough that he can hold his head above water when standing up.

The chlorine in the water will irritate the leeches and you will find them on the edge of the container just above the water line after a few hours.

I do this with snapping turtles when I catch them, I had one small snapper with over 100 leeches on his underside.

As for cleaning the tank, I reccomend you fill the tank with tap water and look for leeches after a few hours, and let it sit overnight.