Here’s Why You Shouldn’t Top Up a Koi Pond with Tap Water

People who own ponds should know about the dangers of adding plain tap water to their ponds. New koi pond owners may not be so knowledgeable.

Can You Top Up a Koi Pond with Tap Water?

So here’s why you shouldn’t top up a koi pond with tap water. There are a huge amount of nitrates in tap water and those are harmful to wildlife. Though not harmful themselves, they harm the environment by damaging the delicate ecosystem of the pond. Nitrates also have a lot of nutrients that can speed up the growth of plants such as blanket weed.

Here's Why You Shouldn't Top Up a Koi Pond with Tap Water. It Hurt's The Delicate Ecosystem

Nitrates enter the water system in agricultural areas from runoffs. Natural, healthy ponds don’t take in water from these run-offs which are why they are healthier. The blanket weed mentioned above is what can grow when these nitrates are plentiful.

Your pond needs flora to develop in order for the ecosystem to be healthy and with blanket weed floating over the top of that flora that development can’t happen. 

Here’s What Happens with Large Amounts of Nutrients In The Water

In this case, they can be harmful. These nutrients can get deposited in the soil. Some of them get firmly planted into the ground and stay for years before they are finally drained back into the water. Others nutrients are run off and get into the water system that way.

The problem with having all these nutrients available is it is a great food source for algae blooms to take ahold. Algae take over your koi pond and block all sunlight from penetrating the waters bottom. Any plants that are underneath this bloom die from not getting any sunlight. No sunlight for plants means they can’t go through photosynthesis.

Then The Algae Bloom Dies 

The algae bloom, that can be caused by the nitrates in the tap water that you added will eventually die. It sinks to the bottom of the pond where bacteria go to work breaking it down. This causes another problem to occur. This process takes oxygen to do so your fish are being depleted of their much-needed oxygen supply when this is happening.

If this oxygen depleting process goes on for too long then larger life forms, such as fish(your koi), will suffocate. it is necessary to get rid of as many algae from your pond as possible through skimmers. You can also get a net to do additional algae removal too.

It is very important to take care of the water in your pond. Things like keeping the pH of the pond in check, making sure all of the filters are working properly, and keeping the cleanest water source available for topping up on a hot, dry day are very important for your koi carps health.

So, now you see why you shouldn’t just top your pond off with just water from the tap. Much tap water is also acidic and can mess with the koi ponds pH balance. All in all you now get why it isn’t a great idea to take water from the sink to add water to your pond. So, what should you do instead?

What is the better option to tap water?

So hopefully you now know some of the risks with using tap water to top off your koi pond. There have been surveys done that say over half of the ponds in Britain are in poor condition and tap water shares a lot of the blame. So you need a different water source.

What is suggested is to use rainwater instead. This will require investing in a water butt to collect rainwater whenever it comes. This water won’t interfere with the natural, delicate ecosystem like water from the tap does. The pH is also much more in balance.

Having a water butt will allow the benefit of having water on hand when there are water shortages such as droughts. Koi love warm weather. They also like their pond homes to stay at least 3 feet deep for moving around comfortably, however.

Some products make tap water safe for fish like the video below shows. You don’t want to just dump tap water in your pond because the results probably won’t be good.

Safe for Humans Doesn’t= Safe for Fish

Human drinking water is ok for human consumption. Our bodies can withstand a lot of abuse compared to what natural pond habitats can. There is also chloramines, which is a combination of ammonia and chlorines, added to water during its treatment so adding tap water to your koi water will require the use of a de-chlorinator as well.

According to a forum I read, a poster on there read a book by the author Lance Jepson that says chronic exposure to 0.002 parts per million of chlorine will cause a fish’s gills to develop hyperplasia of the gill lamellae. ST de-chlorinator is very cheap, so why not use it to make your water instantly safe for your pet fish. ST costs around $7 US. That’s cheap and effective water protection.

Another Problem with Tap Water

As a relier on gov’ment water, you have had the experience of line breaks and repairs. Other things can happen that cause the city’s water department to have to do drastic repairs. At this time, your water isn’t safe and you have a boil order in effect to even be able to use your water.

When this happens the water department increases the amount of chlorine or chloramines in your water to compensate for the larger amount of toxins present in the water. So during this time the amount of chlorine is even higher than usual. That’s another reason you can’t be totally dependant on tap water to keep your pond water level high.


If you thought chlorine was bad for fish there is something even worse-chloramines. You see, the chlorine will stick around for a while, but you can wait for it to evaporate into the atmosphere and it will go away eventually. Chlorine used to be the go-to disinfectant for water companies to make water drinkable, but now more and more water-treatment plants have moved to use another substance- chloramines.

Chloramines are a relatively new compound that combines ammonia and chlorine. The difference with chlorine is that this substance DOES NOT dissolve in the atmosphere. This is great for water companies because they always wanted a way to efficiently treat tap water so it stays treated for a longer period without the treatment product evaporating away. This is NOT GOOD for a koi pond or fish pond owner though.


What about using well water for topping up a koi pond? Now we go to another type of water you might use if you have well water. Well water has an advantage over tap in that it won’t have the chloramines that your city will have. Unfortunately, there is also usually too low of levels of saturated oxygen with well water. Saturated oxygen levels are to koi what oxygen levels are to people. Koi need dissolving oxygen in the water to breath and if they don’t have them then they suffocate.

If we can’t top off with city water, what are we supposed to use? A rain barrel, or water butt, will be able to store up healthy water that your koi carp will love. I like the look of Good Ideas RW50-OAK Rain Wizard Rain Barrel. It has a natural oak barrel look that I think looks awesome and it won’t rust, mold, or rot.

How do you safely remove chlorine from your water? Chlorine needs to be added to our water mains to kill out harmful bacteria before it goes into our bodies. However, you have already read about how these chloramines and chlorines can be harmful to aquatic life. 

So what you need to do is have a very good water filtration system for your pond and add in a product like ST Dechlorinator. I recommend this product because unlike many older water treatment products that only remove chlorine this can remove chlorine and chloramine. It will also remove nitrites and heavy metals from water too. It’s the best dechlorinator product on the market in my opinion.

So don’t go thinking you can just take a hose and fill up your pond…

You now understand about the chemical-laden properties of household tap water and why you shouldn’t top off your koi fish pond with it. You can add it as long as you have the means to clean it out, but rainwater is the best as a natural water source for your fish. Hopefully, after reading this you don’t top off your pond with city water? Leave a comment or question below and I’ll get back to you soon. 

8 thoughts on “Here’s Why You Shouldn’t Top Up a Koi Pond with Tap Water”

  1. Oh! This must be the reason for my uncles loss when he decided to invest in a koi pond. Most of the fishes ended up during. Though it may have other issues but I figured out this is among the reasons since he makes use of tap water as a major supply to the ponds. I’m definitely sharing this post out to him. Thanks so much

    • That sounds like a fantastic idea. Having a good pond pump and a good dechlorinating product would have been something he needed on hand if he was topping his pond up with tap water. That may have been the reason they died off, but there could be many others too. Too high of a pH level could be another possible cause. Tap water is considered acidic.

  2. Excellent article, thank you.  I had no idea tap water could be detrimental to pond fish, though it definitely makes sense now that you mention it.  I also had no idea that they were moving away from chlorine in tap water to ammonia and chlorine.  What about the fluoride added to a lot of tap water in the US?  I’m uncertain about the UK or other areas, but fluoride is added to our drinking water here. Is it not a problem elsewhere?  And if it is, does it cause problems for pond life?  I’m just curious!  Thanks for the post!

    • Sorry, forgot to mention it, but fluoride is another chemical added in to our tap water in the US too. I think it is a universal thing for safety concerns. They use it in order to prevent tooth decay, but it is opposed by many for a possible link to cancer. That being said, fluoride levels are regulated and at those levels they don’t seem to adversely affect the health of wildlife.

  3. Good stuff to know! it makes sense too, as I read through! the content is straight forward and very insightful, thanks for the post and gives detailed information that I would have never thought of.  Does this information stay strictly toward any other underwater aquariums as well or just koi? 

    • Tap water is just plain bad for all wildlife. This includes fish in aquariums, ponds, lakes, and streams. Thank you very much for reading.

  4. hi, very interesting blog.  I know nothing about koi pond but your blogs has a lot of info. about the ponds.  The video about ponds is very helpful for those not to knowledgeable about ponds.  How long have you been building these ponds.  Would be nice if the blog explains what koi is from the start.  Nice page about these ponds.


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