Pond Water Quality

Keeping healthy Koi and Goldfish begins with good pond water quality. This page contains information on what is required and how to maintain and monitor your water. Koi and Goldfish health is also highly dependent on disease management, nutrition and environmental conditions. Therefore, we have put together a page covering each of these issues. Visit them at the following links:

Water Clarity

Besides algae, the most common problem that occurs with Koi Ponds and Water Gardens is poor water clarity. Whether the problem is green, brown or cloudy water, the solution almost always boils down to maintaining a clean pond, cleaning your biofilter PROPERLY, and making sure that you are pumping the correct amount of water through a properly sized and designed filter system.

Maintaining a clean pond involves a lot of prevention. When you set up a pond you want to be sure that runoff from the lawn and/or landscape is not washing into it. Runoff water from rainstorms is notorious for carrying loads of nutrients that are picked up from soil, fertilizers and organic material in mulch and compost. However, even a perfectly designed pond will pick up a certain amount of debris from fish waste, aquatic plant foliage, leaves blowing into the pond and dead algae. You can minimize these issues by not overfeeding the fish (many pond owners do not feed the fish at all!), pruning aquatic plants, installing Pond Netting in the fall, and keeping algae under control. Once an inch or so of debris has inevitably built up on the pond bottom, it is beneficial to remove it in a way that does not disturb the balance of the pond. We recommend not having stones in the deepest portion of the pond so that you can easily clean this debris out with some sort of Pond Vacuum without having to remove the fish and drain down the entire pond. A complete pond cleanout is actually quite hard on the pond and usually results in weeks or months of wild water quality swings. We recommend not to do a complete pond cleanout unless the pond gets away from you. If you must do one, limit it to once in the spring every other year.

Cleaning your biofilter properly is critical to maintaining good water quality. Most types of pond filtration systems have prefilter pads that remove debris and then bags of media (lava rock, bio balls, filter floss, bio media, et cetera). In general, the bags of media are your biofilter and they function because of colonies of beneficial bacteria that live on them. When you clean your filter, you should thoroughly clean your filter pads, but the bags of media should not be cleaned unless there is debris clogging them. In that case, they should only be cleaned with pond water. To clean bio media, we recommend setting the bags in the grass and dumping a bucket of pond water on them to knock the debris loose, and then get them back in the water! Furthermore, we recommend that you move the bags of media into the pond in the winter so that they never dry up, and then you can move them back to the filter box in the spring, alive and well, and have instant filtration. So, if you never temperature shock and never dry out your bags of media, they will be infinitely more effective at cleaning your pond.

Finally, proper water flow and filtration setup is critical. For more information on the topic, please see Pond Design Requirements.


Alkalinity and pH

Fortunately for us in the Upper Midwest, our well water generally contains sufficient Alkalinity to buffer the water, which keeps the pH stable, requiring little conditioning.  However, city water can be a different story! It MUST be treated to neutralize Chlorine, Chloramines and Heavy Metals before adding fish, and Alkalinity should be monitored as well. To cover all possibilities, we recommend initially treating all pond water with a Water Conditioner at least an hour (preferably 24 hours) before stocking with fish. Small amounts of make-up water usually does not need to be treated, but you should re-dose with the appropriate amount of water conditioner when performing water changes (see below).

Those of you in parts of the country that do not have water that is buffered as well need to pay particular attention to pH and Alkalinity and adjust accordingly. There are various Test Kits and Water Conditioners available for you to do this with.

pH and Alkalinity are important water quality parameters, but they are not the only ones. Below is general information on the most important water quality topics.


This is basically a measure of Hydrogen ions in the water. Generally, we recommend that you maintain a pH between 7.0 and 8.5. Lower pH can result from bacterial activity in the biofilter consuming carbonates from the water, which reduces Alkalinity, which will eventually lead to a pH crash. By performing periodic water changes and treating your water with a Water Conditioner, you can avoid this problem. However, if your test kit indicates pH below 7.0 and the fish swim erratically and respire rapidly, a sudden pH crash (Acidosis) may have occurred. In this case you should dose the pond with pH Up until the pH has raised to around 7.5. High pH levels above 8.5 can occur, especially in ponds with a lot of aquatic plants. If your test kit indicates pH of 9 or above and your fish have pale gills and show areas of skin erosion with increased mucus production, you should decrease the pH by adding an appropriate amount of pH Down until the pH has lowered to around 7.5. Please note that many ponds with large amounts of aquatic vegetation see increases of pH through the day. Therefore, it is a good idea to check your pH levels both in the morning and afternoon. If they are normal in the morning, but high in the afternoon, you don’t need to worry about it, unless you are having fish issues. There are various test kits and strips available on our Meters and Test Kits page.


Alkalinity in pond water is the great equalizer! If a pond has sufficient Alkalinity the pH will hardly budge. Traditional wisdom is to maintain the Alkalinity at or above 25ppm, but our opinion is that if you are under 50ppm, you are getting close to a potential crisis. To improve Alkalinity, we recommend adding a Water Conditioner to all pond water initially and re-dosing the appropriate amount whenever you perform a water change. Alkalinity can be monitored with various test kits and strips available on our Meters and Test Kits page.

Dissolved Oxygen

A common myth is that because Koi are actually Carp, they can survive for days with practically no oxygen in the water. The exact lowest level that will kill Koi and Goldfish is tough to determine because it depends on size of fish, temperature, duration of exposure and many other variables, but suffice it to say that the Dissolved Oxygen levels should NEVER drop below 4ppm. Another myth is that you can throw a bunch of “Oxygenating” underwater plants into the pond to take care of your oxygen levels. Plants do give off oxygen in the presence of sunlight, but when the sun goes down, plants actually use oxygen, and lots of it! In fact, the more plants, algae and organic debris that you have in your pond, the more supplemental aeration that you should have. To maintain adequate oxygen levels, you should NEVER turn your water pump off at night – this is precisely when your pond needs aeration the most! Also, if your pond is prone to sagging oxygen levels, consider installing a supplemental Aeration System, which can also be used for highly efficient winter de-icing. We recommend that you maintain dissolved oxygen levels above 5ppm. The best way to monitor Dissolved Oxygen is with an Oxygen Meter, which is available on our Meters and Test Kits page, but is fairly pricey. Test your oxygen at or just before sunrise, as this is the time when oxygen levels are lowest.


Ammonia occurs in ponds as a byproduct of aquatic animal defecation, respiration and the microbial decomposition of organic debris. The toxicity of ammonia is highly variable, depending mainly on the pH and water temperature. Basically, there are 2 types of Ammonia, Ionized and Un-ionized, and which one that is prevalent depends on pH and water temperature. Un-ionized Ammonia (toxic ammonia) is most toxic and occurs increasingly as pH and temperature goes up. Ionized Ammonia (Ammonium) is less toxic, and increasingly occurs as pH and temperature goes down. Un-ionized Ammonia levels should remain below 0.01ppm. If levels go above this you should stop feeding the fish, do as large a water change as practicable, treat make-up water with a Water Conditioner and then add Pond Bacteria to the water to reseed your biofilter. It is common to experience high ammonia levels in newly established ponds or in ponds that shut down their biofilter over the winter, but if you constantly have problems after the pond has been established for 2 months, you may be killing your biofilter when you clean it (never let it dry out or expose it to different water temps than what it is in), or over-feeding your fish, or not cleaning debris from the pond. Signs of Ammonia toxicity include smaller fish laying on their sides on the bottom or respiring rapidly at the surface. Check the Meters and Test Kits page for options on testing for Ammonia.

Nitrite and Nitrate

Nitrite is a byproduct of the microbial decomposition of Ammonia, and Nitrate is a byproduct of the microbial decomposition of Nitrite. Of the 2, Nitrite is by far more toxic, but both can cause real problems. The bacteria that reduce Nitrite to Nitrate are slower to develop in biofilters than the bacteria that reduce Ammonia to Nitrite, so there is generally a spike of Nitrite following the start-up of a biofilter. Nitrite should be kept below 0.01ppm. If levels spike above that you can temporarily remedy the situation by adding Water Conditioner Plus or 1 pound of Pond Salt per 100 gallons of pond water, but the long-term solution is the same as described above for Ammonia.

Nitrate accumulates over time in ponds because anaerobic bacteria are required to convert it into Nitrogen gas, which dissipates naturally from water, and anaerobic bacteria are hard to come by in properly managed ponds. Nitrate should be kept below 100 ppm. You can generally prevent the pond from attaining this level by performing routine water changes. Check the Meters and Test Kits page for options on testing for Nitrite and Nitrate.

Water Changes

One of the easiest ways to improve water quality is to perform a partial water change. Generally, a 10 to 30% change is good enough to improve water quality and not impact the biological filter. To do this, simply pump or drain roughly 1/3 of the water out of your pond, add a Water Conditioner (the appropriate amount for the volume of gallons that you removed from the pond) directly to the pond water and refill the pond. It is a good practice to combine pond cleaning with your water change. See the Pond Equipment and Maintenance page for equipment. We recommend performing this procedure once a month if you have a buildup of debris on the pond bottom, or whenever you are suffering a water quality or fish disease problem. Just be sure not to change your water temperature by more than 5 degrees in a day.