I'd add dechlorinator regardless. I think 3 days would be enough with out.

Having lost an entire pond's worth of fish due to city water...I'm more cautious.

I agree with Groundbeef. To be on the safe side I'd add a dechlorinator. I may be wrong, but I vaguely remember hearing that, while the chlorine evaporates, there are some chemicals that don't; chloromides or something like that.

Anne

the worst thing is to drain your pond.water takes a long while to age.you need to balance the pond with plants, not over feed your fish,and do a 10% water change and clean your pads each week.how big is your filter and pump

use a water conditioner, better than a dechlorinator.
here is one Ultimate

I have never used it. I only have small ponds that generally sit a week or two before I add fish!
Here is one with some Rosy Reds in it. Don't look for them as they hide in the jungle of plant growth!!
{{gwi:230670}}

Joe,
You are right & draining was the last thing I wanted to do for that reason. Something is off in my pond that I can't figure out why. My bio filter is 50 gl skippy with water hyacinth floating on top. When I drained and cleaned the pond the filter media was not that dirty and yucky. I have a Sequence 750 3600gpm pump. I also have a small bog with floating hearts, watercress and elephant ear. I have about 5 hardy lilies and 1 tropical lily that has been blooming all summer long.
I haven't really been feeding my fish cause they can't see me or me see them.
This is not the usual green water/pea soup seasonal change. I know patience works best for that. It's green and rather murky with a couple inches of clarity and has been most of the summer. I noticed it started going green again late yesterday.
(Now our weather here is usually sub tropic and with this darn drought it's more desert like. I wonder if that is playing a part. When I say hot, we broke records with 40+ days of over 100'.)
I am thinking of taking my bio filter from my small pond and connecting it to the large pond in hopes of getting a jump start on the beneficial stuff and help combat the bad stuff that is taking over. Thoughts??

berlyn, it sounds like you are doing everything right.it sounds like the heat might be too much.have you tried a uv light. i would buy one rated larger than the pond, like the 36 watt tetra.that should kill the algae.also i never feed my pond plants my water is clear all year.one more thing all that heat a 1100 gal pond might have gotten to warm.

Don't know what your city water contains but mine has both chloramine and chlorine so must be treated as the chloramine will not break down without treatment and will kill fish very quickly. You also didn't say how many koi you have in your pond but a 50 gal skippy filter doesn't sound big enough for 1100gal pond. I had the opposite problem of you this year very cold spring and summer with only a few days above 75F so my plants were slow coming and had algae all summer long. Mine was because didn't have enough plant coverage and high phosphates in my city water. Tried some laguqna phosphate remover this year and it seemed to help.

I have 3 8" Koi in my pond. I did add a water conditioner that removes chlorine & chloramine and nuetralizes heavy metals yesterday.
I also have a bog in addition to my skippy filter. I've never tried a UV filter, as I never have had a problem like this in my 10+ years of ponding.
Can you test for phosphate??

@Berlyn

1. Yes you can test for phosphates. Use a liquid test kit, the 'strips' are for crap.

2. You might also want to check your PH. I suffered a terrible crash earlier this year. Ended up buffering the water w/ some arm N hammer baking soda, then added some crushed oyster shells. This can help if you are having high heat/temp fluctuations. Keeps the PH more stable and it's easier on the fish.

I believe after much debate with my better half, I have figured the problem out. I will see in several days if this was the root of the problem or not.
Luckily half way through this "process" (on day 2 of this), I grabbed my camera phone & snapped some pics.

I will post them later, when I read, how to do that.
Right now, I am too pooped & sore!

Chloramine and chlorine will most likely react with stuff in the pond before it gets a chance to evaporate. Even a just cleaned pond still has a lot of stuff for the chlorine to react with. A chlorine test will tell you exactly when it's safe for fish. Much cheaper long term than a dechlorinator or having to buy new fish.

Ammonia is a by product of chloramine breakdown. Most dechlorinators convert the ammonia to non-toxic ammonium. Unfortunately ammonia tests see both forms as the same so there's no way to tell if ammonia is at a safe level unless the total is zero. So I think it's prudent to test ammonia and wait for a zero level.

Lastly, depending on your city, testing KH and adding buffer if needed, is also prudent as well as easy imo.