Approximately how many gallons is your pond? Depending on the size you could probably just add water out of the faucet to top off. A call to your water provider and they will tell you their chemical additives.

by my 'best' figuring its @1200 gallons (100 gal skippy filter, 165 gal intermediate level preform, and a 13'x5' kidney 'in the ground' with a max depth of 4' and shelves in 18" shelves so I get all turned around in accurately figuring volume.
I'd be happy just turning on the hose sprinkler for a bit every night if the 'bad stuff' will dissipate during the sprinkling.
Will

If your "bad stuff" is just chlorine you will not have any problem just topping off right out of the hose. 20-40 gal. will have no effect.

Chlorine is a gas and will dissipate into the air. Sunlight and heat will also kill chlorine. I'm not a ponder but what I would do is fill my 30 gallon container and let it sit over night or put it in the sun and still leave it out over night. The next day I would add the water to the pond and start the process over again. I wouldn't be adding any other chemicals to the water for chlorine. Also, you can purchase a chlorine test kit to check it if you want.

As far as a sprinkler goes, I don't know if that would be enough. Get a test kit and collect some water from the sprinkler and check it out.

When my kids were younger and had goldfish, when it came time to changing the water, I never treated the water with chemicals. I boiled it, waited for it to cool, then changed the water. Never had a problem.

How are you treating the water now?

I had been using the 'Step 1' solution which is supposed to dechlorinate tap water. I experimented last week filling my rain barrel (55 gal) leaving it 24 hours and then letting it run into the pond...didn't seem to have any effect on fish or plants. Also, did water the plants with a big 'fountain' sprinkler which covered plants, pond and all for about 30 minutes another day and didn't seem to bother...but thought I'd check for the experience of those here.
Will

I top off my pond all the time with water from the city, with no additives. In a pond your size, you should be fine. I turn the water on very slowly and then set the alarm on my cell phone for 1/2 hour. Its all good. Makes life much easier.

I've got a water line direct to pond with either a Melnor or a Gilmore water timer. Just turn the dial to desired time and forget about it. They are mechanical and not very expensive.

fred

I just use city water direct from the faucet. I know it has chlorine, but I never have any problems and I have a lot of evaporation at this time of the year so I'm topping it off pretty often. I would say it drops about 4 or 5 inches before I refill. I guess my pond would be about 500 gals. Maybe a little less. Good luck. BT

The biggest problem with topping off a pond using untreated city water is if they city decides to "super-charge" the water lines, as they do frequently. If there is construction anywhere along the lines and they have to break into a line, they will inject a super high dose of chemicals to handle any possible contamination. If this has happened, you will not even know it until the fish are accidently killed. chemicals made to neutralize chlorine and ammonia are not expensive. It is just unwise to take chances and not dechlorinate water prior to use.

You can check with your water company to see if they use chlorine or chloramines. Chlorine is essentially gone from water within 24 hours. Chloramines, in contrast, can remain in water for weeks.

Plants, I think are fine with either, it's the fish that are the problem--both of those chemicals are toxic to fish. Do you have fish, or just plants?

Check into Sodium Thiosulphate as an inexpensive dechlor. It's the cheapest way to go.

I use a charcoal filter and jar(like the one you use for under the kitchen sink)and used brass fitting hooked to each side of the filter jar with quick cupplings . I hooked it to my hose and ran a 6 ft.long from the other side of the filter jar. I only change the filter every 6 months. Easy to hook up and fill my pond. Hope this makes sense. Works great!

Products to treat your pond aren't that expensive and why risk your fish even if they weren't expensive. I gave my daughter two small goldfish and a 10 gal tank for my grandson. She did a water change 20% but didn't read the instructions on the dechlorinator/dechloramine I gave her. She should have used a teaspoon but only used a couple of drops because she thought she didn't change out that much water. Right after doing the water change the fish sat on the bottom of the tank. The next morning they were dead. Our water here is treated with both chlorine and chloramine. I use my sc for water changes as can shut it down and fill, treat with Prime then run again. The water system here was changed out two years ago to both chlorine/chloramine--previous only treated with chlorine. Within a month over 25% of the households in town had to replace their hot water tanks--something with the new chemicals blew alot of tanks--mine included so who knows what else is in there. I also use koi clay which is supposed to remove harmful metals in the water.