Question
Is there a chemical I can put in to kill leeches in our pond?  We do not want any animals ie: fish either, but do want it to be safe for swimming.

Answer
The best I have come across is on Squidoo and which was successful it is as follows:
(http://www.squidoo.com/how-to-remove-leeches-from-your-pond-without-chemicals)
The Search For Safe Leech Control
Avoiding Dangerous Chemicals.
Hmmmm.... how to kill leeches in pond..... If you want to go down the chemical route it is complex and dangerous. It is also expensive to buy pond chemicals in the quantities needed. It involves replacing all plants, estimating exactly how much water the pond holds, mixing chemicals that could possibly harm the smaller fish - and destroying every living invertebrate in the pond. That was not something we were prepared to consider. So what is safe to use to kill leaches? I searched and searched until I found the idea of constructing a leech trap out of a coffee tin. Here coffee tins are not so easy to find, they tend to be metallic cardboard. Then I thought of fresh soup containers and the leech trap was born.

At present we are on our third trap and numbers are reducing. I was horrified at the number of leeches in the last trap.... How you dispose of them having caught them is another issue of course, but you need to remember that leeches are dangerous, but they are also living animals with a nervous system and should be disposed of humanely. Freezing them is one method, or release into a river system, if it is permitted, where larger fish may eat them.
Making the holes in the leech trap
Boring from the outside of the leech trap
Boring holes to make a leech trap

Boring holes from the outside to create a burr on the leech trap makes it less likely leeches will crawl out through the holes. These holes can be made carefully with a steak knife and should vary in size from 2-5mm.
Raw Materials for the Leech Trap
And how the Leech Trap works
The leech trap baited with kidney.

The leech trap works on the same principle as Winnie the Pooh and the Honey Jar. Leeches, being greedy, will gorge on blood and when they do they become so big that they get stuck in the hole. That is the principle - and it works. The main items needed are meat and a container. Liver or kidney is cheap and works well. Here you see a view of the inside of the leech trap. The holes are also burred which discourages early escape.
Sinking the leech trap
Placing the leech trap in your pond
Leech trap Floating in the Pond

Just use a net to lower the trap into the water. It will fill with water and float upright. This will catch many leeches as they can sense the meat and swim up to it. To sink the trap deeper, put stones in before the meat. Then just leave the trap for a few days to a week. It will soon fill if you have a bad infestation.