Using Well Water To Fill Your Pond? Everything You Need To Know

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As a pond owner, you might get frustrated when your pond’s water level drops dramatically since it threatens your fishes’ survival. Perhaps you’ve considered refilling your pond with well water to solve the problem.

If you are using well water to fill a pond, you should first perform a standard water quality test to test whether the well water is compatible with your pond water. Water quality can make or break the health and reproduction of your fish. 

In this article, I’ll discuss in depth why using well water for your pond may be dangerous. I’ll also show you what to do to solve this problem. Read on!

Why Well Water Is Not Good for Ponds

The short answer to the question “why well water isn’t good for ponds” is that it usually contains a low concentration of oxygen. Moreover, due to surface runoff, it may sometimes have high levels of salt and other contaminants as well — meaning the water’s pH is adversely affected. 

The long answer necessitates a detailed explanation of the origin of well water.

Well water originates from underground sources called aquifers. Essentially, these are waterlogged bodies of porous rocks and sediments. 

Aquifers typically get their water from natural sources such as rain and melting snow. Rainwater percolates from the surface through the tiny spaces within the soil toward the absorbent rocks of the aquifer. When wells are dug, the aquifer channels water toward them.

As water seeps from the surface into the aquifer underneath, it absorbs and carries the minerals found in the soil. As a result, well water accumulates high concentrations of salts and minerals, making them “hard.” 

The degree of salt concentration, however, is highly dependent on the location of the well.

For example, if your well is near a farm, it’s likely to be fed with water contaminated with fertilizers and pesticides from the soil. Fertilizers contain salts like phosphates, sulfates, and nitrates, which can poison your fish. Moreover, they accelerate the production of algae and other aquatic plants in the pond, reducing the amount of dissolved oxygen therein.

The same applies to wells in industrial areas. Waste materials are likely to mix with rain water and drip into the ground, polluting nearby aquifers and any wells that source water from those aquifers.  

Additionally, well water holds higher concentrations of dissolved carbon dioxide than above-ground water. Combined with the high concentrations of salts and minerals, this greatly affects the pH of well water. 

Therefore, if you use well water in your pond without treating it, your fish may be unable to handle both the extremely low levels of dissolved oxygen and the high levels of minerals that can harm them. 

For those reasons, it’s vital that you test and treat your well water to match that of your pond. 

How Well Water Is Tested and Treated

Testing and treating well water involves three basic steps:

  1. Sampling the well water to check its quality
  2. Testing for crucial factors such as pH, TDS, and phosphate and nitrate levels
  3. Treating the well water

Whether you’re taking these steps yourself or hiring an expert to perform them for you, below are the steps to test and treat well water.

1. Check the Well Water Quality

Before treating well water, you need to sample it first to get a better idea of its quality and the type of treatment it needs. 

There are three ways you can do this:

  • Dip your test materials or equipment directly into the well water. This method provides the most immediate and accurate results since the water is tested undisturbed. However, it’s possible for the testing materials (e.g. a bucket) to further contaminate the water.
  • Draw water from the well and test it in a container. This is the most appropriate way of testing if you’re using chemicals.
  • Have it tested at a laboratory by a professional. This method will give you the most accurate information. The only downside is that you have to take the utmost care in preserving the sample so that its quality doesn’t change while in transit to the laboratory. 

2. Test for pH

pH (Power of Hydrogen) refers to a substance’s degree of acidity or alkalinity. This is measured via a scale that consists of values 0 to 14. For example, a neutral substance such as pure water has a pH measurement of 7. Acidic substances have values lower than 7, while alkaline ones have more than 7.

The appropriate pH of pond water should fall between 6.5 to 8.5. Anything lower or higher than that will stress your fish. There are three ways to test for pH levels in the water, which you can learn more about here.

3. Test for Total Dissolved Salts (TDS)

As mentioned earlier, well water tends to have high levels of salts. Therefore, it’s essential to measure the salinity of the water, since different aquatic species have different levels of salt tolerance. Salinity is usually quantified in parts per thousand (ppt) or percentage. 

If your pond is made of fresh water, the salinity value should be less than 1 ppt. If the water is salty, salinity values will range between 34 to 36 ppt. The salinity of the well water should match that of your pond for your fish to survive. 

To learn more about ways to measure the salinity of water, read this article.

4. Test for Phosphates and Nitrates

Last but not least, you should determine the levels of phosphates and nitrates in the well water. Fortunately, you can easily purchase test kits for this purpose. 

I use the API Reef Master Test Kit (available on This master kit consists of seven solutions for testing not only phosphates and nitrates but also carbonates and calcium. Also, it has two color cards and four glass test tubes to hold the solutions safely. This kit gives fairly accurate results, and it’s easy to use as well. 

5. Treat the Well Water

If, after conducting the above tests, the well water quality turns out to not match your pond’s, you should treat it first.  

There are multiple ways to treat water, but the best is arguably the Reverse Osmosis (RO) filtration method. This method ensures that all contaminants are eliminated from the water, making it pure. If you want to treat your well water using an RO system, choose one that features five or more stages of filtration.

The only downside of using RO is that it removes even the salts that are essential for fish. Therefore, you should add these salts to the water after it’s purified.

Additionally, you’ll need to correct the pH of the water. Once you’ve pumped the treated water into the pond, you should add a PH adjuster as soon as possible. 

There are numerous ways to adjust the pH of your pond. For instance, you can order the Ecological Labs Microbe-Lift 7.5 pH Buffer Stabilizer from This product will safely adjust and stabilize the pH of the pond water to about 7.5, which provides optimum conditions for fish survival. It’s also formulated to maintain the osmotic balance of the pond (the balance between salts and water).

On the other hand, an aeration system can also help treat the high levels of carbon dioxide in your pond. Essentially, aeration systems increase dissolved oxygen concentration and adjust the pond’s temperature to optimum levels.


Untreated well water isn’t safe for fish, as they are very sensitive to changes in water quality. To use well water for your pond, you’ll first need to test it. If the quality doesn’t match that of your pond water, treat it by purifying, aerating, and adjusting its pH.

Also, make sure to keep an eye on your pond, as its chemistry can become drastically altered when you’re doing things like changing the water.

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