I use Kockney Koi Pond Dechlorinator which is available on Ebay. It was recommended to me on here, can't remember who by.
Now heres the clever bit, its exactly the same stuff as aquarium dechlorinator, just more concentrated because who would want to buy 40 bottles of Stress Coat to dechlorinate their pond? Nobody!
I use 1ml of dechlorinator for every 75 litres of water, and off the top of my head, i think you would need 20ml of Nutrafin dechlor for the same amount of water.
Anyway, i got a 500ml bottle from Ebay, i have a 260 litre tank which i do 75 litre water changes on every week, so the 500ml bottle will last 9.5 years, and it cost £8. Beat that for value!
It removes chlorine, chloramine, heavy metals etc as most dechlors do, its just that pond dechlor is cheaper because nobody would pay £150 to dechlorinate their pond.