Steve:
Other posters must think we're nuts, we live on the same Island, belong to the same koi club, am only a phone call away, and yet we're corresponding on the Internet through Japan... Duh!
Guess I may not get any more responses since you seemed to cover most of the bases...
Regarding your quote:
"To be safe, you would want to replace the activated carbon every now and then"
I agree 100%... The cartridges only cost $8.99/ea from the major distributor, and are supposed to be available at a number of the big box stores...
When you figure the losses that a number of posters on this board alone have suffered, changing the cartridges at $8.99/mo. is a pittance...
Howard H is supposed to have lost 10s of thousands of dollars worth of koi, a month or so ago after a major water change, due to chlorine... In addition the Kingyo lady in Kaimuki told me that she lost between 2-300 ranchus to chlorine poisoning from her tap water...
So monthly changes it is....I'd rather throw $9.00 away each month, than suffer the loss of just one of our koi...
Aloha and Mahalo, Mike