City Water & Koi Ponds: What Every Koi Keeper Should Know

If you keep koi and use city water to fill or top off your pond, there’s something important to keep in mind: the water coming from your tap isn’t always exactly the same throughout the year.

Municipal water is treated to make it safe for people to drink, but some of the chemicals used in that process can be dangerous to koi and other pond life. This is especially important during periods when your local water department is flushing water lines or temporarily changing its disinfection process.

At Koi Boyz, we believe water quality is one of the most important—and sometimes overlooked—parts of keeping healthy koi.

Why City Water Can Be Dangerous to Koi

Most municipal water systems use chlorine, chloramine, or another disinfectant to control harmful microorganisms in drinking water.

That’s good for people, but not necessarily for your pond.

Chlorine can damage a koi's delicate gill tissue and protective slime coat. Chloramine can be particularly troublesome because it is more persistent in water than free chlorine and doesn't simply disappear as quickly by allowing water to sit.

The danger becomes greater when a substantial amount of untreated city water is added directly to a pond.

Watch for Municipal Water-Line Flushing

One situation koi keepers should pay particular attention to is periodic water-system maintenance.

Cities and water utilities periodically flush their distribution lines to remove sediment, maintain water quality, and keep the system functioning properly. Some utilities also periodically change their disinfection practices—for example, temporarily switching from chloramine to free chlorine.

These changes don't automatically mean the tap water is unsafe for people. The water utility is treating drinking water according to applicable standards. Koi, however, are much more sensitive to chlorine and chloramine than humans are.

That's why a change that may be perfectly acceptable for drinking water can still create problems in a koi pond.

Don't Assume Today's Tap Water Is the Same as Yesterday's

One of the biggest mistakes a pond owner can make is assuming:

"I've always filled my pond from this hose and never had a problem."

Your municipal water conditions can change.

If your city announces flushing, maintenance, or a temporary change in its treatment process, take extra precautions before performing a large water change or adding significant amounts of tap water to your pond.

During these periods, consider testing the source water for free and total chlorine and make sure your dechlorinator/water conditioner is designed to handle the disinfectant your utility uses.

What Koi Keepers Should Do

If you rely on municipal water, a few habits can greatly reduce your risk:

• Always treat city water before or as it enters the pond. Use a pond-safe conditioner capable of neutralizing chlorine and, when applicable, chloramine.
• Check your local water provider's notices. Look for announcements concerning hydrant flushing, water-main flushing, disinfection changes, maintenance, or other treatment changes.
• Be especially careful with large water changes. The more untreated tap water you introduce, the greater the potential exposure.
• Consider keeping chlorine test strips or a suitable chlorine test kit on hand. Don't rely solely on smell or appearance.
• Watch your koi after a water change. Unusual gasping, gathering near waterfalls or aeration, erratic swimming, lethargy, or other sudden behavioral changes should prompt an immediate water-quality check.
• Maintain good aeration. Healthy dissolved-oxygen levels are important at all times, especially when fish are under stress.

Remember: "Safe to Drink" Doesn't Mean "Safe for Koi"

This is an important distinction.

Municipal water is treated for human consumption, not for maintaining an aquatic ecosystem containing fish, beneficial bacteria, and plants.

Koi live in that water 24 hours a day and continuously pass it across their gills. Even relatively small water-quality mistakes can therefore have consequences that aren't relevant to someone simply drinking a glass of tap water.

A Little Prevention Goes a Long Way

Don't be afraid of using city water—thousands of successful koi ponds rely on it every day. The key is treating and monitoring it properly.

Pay attention to notices from your local water utility, properly condition new water, test when appropriate, and be especially cautious when your municipality is flushing lines or changing its treatment process.

A few extra minutes of preparation before a water change can help protect years of investment in your koi.

Healthy water = healthy koi.

— Koi Boyz
Purveyors of Aquarium and Pond Fish