There are many ways to treat tap water. Your method is one, and it's great for
such a large pond. Anyone that has a large pond and want to follow Roger, buy a
cheap gallon of Sodium Thiosufate. It's the fixer used in processing film and
x-ray.

Some of us have much smaller pond/aquairum. We don't want to mess with chemical.
So, we prefer to filling and topping off from filtered tap water. The great
thing about this method is that the chlorine and chloramines are removed from
the water, before the water contact the fish.

One method that I sometime used is putting potassium chlorine in the pond. I
then turn on the hose to fill up. Either the Cl- for the K+ block the absorption
of chloramines. I forgot which one does that. I don't have access to my
references at the moment.

roger wrote:

I've been reading all of these responses to the question of topping up
water.

Sure a filter might be a great thing. But I use a simple an inexpensive
product
which I picked up from a local chemical company. I read about it
in a book I borrowed called "KOI HEALTH AND DISEASE" by
Dr. Erik Johnson. This is by no means an attempt to advertise a book for
a friend or relative. I read the entire book, and picked up many tips along
the way. Among them I found a way to make an inexpensive salt tester
for testing the levels of salt you've added for treating disease(s), as well
as how much salt to use for treating disease. Another thing I learned is
that the water softener from Home Depot is perfect for treating fish
disease.
Not sure what I payed for it, but the savings are amazing compared to what
you'd pay to treat using conventional salt treatment kits from local pet
stores...
The last tip I will tell you about, (yes their were more) is how to
make a batch of dechlorinator for your pond or aquarium.
I purchased a bag of sodium thiosulphate from a local chemical
supply shop. Mixed with water and voila. Easy as pie, easier
and it works great. This is the same stuff you buy in local pet
shops for a small fortune when you consider the sizes of some
of our ponds. My average mix is about the size of 2 coke cans.
This is good for 20,000 gallons. I purchased a 50 lb bag for about
$60.00 it will be around long after I'm gone. Seriously, this stuff is
mixed in such small doses it isn't funny. But honestly I won't
need to purchase another bag in my life time, even if I live another
50 years. 6,500 gallon pond.

Roger