A bit late for an answer,but Manitoba Wendy you likely live in an area that has yellow perch. Catching 4-5 of them in a local lake and dumping them in your pond will solve your leeches issues, and they won't be nibblers either!
We did the same here with our kids, they caught the fish, introduced them in our pond and have not seen any leeches ever since. Your pond is too small to try walleye IMO.
Drawdowns are also a good advice for a swim only pond usage where no aquatic plants or fish are wanted.
Leeches travel on land during warm rainy nights to find isolated ponds to find frogs/blood for their reproduction. Only female leeches will suck blood.

Mario Paris,
Fish & Wildlife Management Technician, CEO of Canadianponds.ca Products