Greg Cooper wrote:

My book on Koi care written by Doc Johnson says that there is not safe
level for Chlorine exposure. Only amounts that are not immediately
lethal. Chlorine does cumulative damage to fish gills.

That's true, but...

Chlorine is extremely reactive - that's why it's used in the first place.
It will get bound to all sorts of things in a pond (creating carcinogens in
the process...) and I really doubt it has much chance to get to fish gills
if you use small waterchanges. I also wonder how long the chloramine bond
can hold up in a pond loaded with organics.
--
derek