leeches in upper lily pond

Question
I have a 17,500 gallon koi pond (converted swimming pool) and an upper lily pond with circulating water from a waterfall/stream through the lily pond in to the big koi pond.  I have many very small leeches in the upper lily pond.  Just today while cleaning that upper lily pond I had 3 leeches attached to my hand.  Are they a serious problem for the koi?  If so, what do I do to get rid of them since I assume they have circulated through my bio filter, pumps, etc throughout my pond?

Answer
Hi,
They will not do your koi any good and I would treat the water to get rid of them I don't really know the name s of any products to do it in the US as I live in the UK but there is products on the market to do it, visit your local aquarist shop they should stock something.
George.