The Components of Our Fish Pond


When we look at our ponds, most of us might simply see a body of water with fish swimming in it.  But what’s comprised in that water is much more than just that.  Many entities factor into what makes our water a dynamic medium for our beloved fish to live their lives.   Each one of these entities is dependent on one the other more than most know.  Let’s take a look at each entity individually to better understand its needs.

Nitrifying Bacteria

While there are many forms of bacteria present in our ponds, the nitrifying bacteria are the bacteria (Nitrosomona & Nitrobacter, to be exact) that we encourage to grow in our biological filter, or bio filter.  These “beneficial” bacteria are responsible for converting the ammonia into nitrite and then later into nitrate, which can be consumed by plants and algae. 

The population of these bacteria will increase as the ammonia levels increase, given there is sufficient surface area and oxygen present for their survival.  Likewise, as pond water temperatures decrease to 50⁰F or food supplies decrease, these bacteria will become dormant. 


If you were to test the pH of your water throughout the day, you would most likely note differences.  For example, morning pH levels are most generally lower compared to late afternoon pH levels.  The pH swings will depend on the buffering capacity, or the ability to resist changes in pH, of the water.   There are buffering products available to help with these large pH swings, if you have a problem.  

All living things in the pond are affected by pH.  Large fluctuations in pH or highly acidic conditions can stress or kill your fish.  In addition, nitrifying bacteria prefer alkaline environment in order to complete the nitrification process.  The higher the pH level, the more toxic ammonia becomes.  As you can see, a healthy and stable pH plays a key role in the overall health of your pond.


It should stand to reason that the pond temperature will usually be the lowest in the morning and highest at sunset.  Pond temperature is affected by the amount and intensity of sunlight, prevailing winds, heat loss due to lack of plants covering the surface, etc.  

A good rule of thumb is that the temperature shouldn’t fluctuate over 9⁰F in a 24 hour period.  Keep that in mind if moving koi between different temperature environments.  An upward temperature movement is tolerated more than a downward movement in temperature.


Oxygen, in the form of dissolved oxygen, is probably the single most important aspect of our pond and yet is so easily looked over.  All living creatures in the pond rely on it, including: the fish, the nitrifying bacteria, the plants and even organic decomposition.  Oxygen is introduced into the pond when the water comes into contact with the atmosphere, as well as, through pumps, aerators, waterfalls, etc.

Oxygen levels can fluctuate drastically during the day and night, as well as, in hot and cool temperatures.   For example, your pond’s oxygen levels can be much lower at night due to the plants actually consuming oxygen instead of producing it, like it does during the day.  Another fluctuation example is that warmer water doesn’t hold dissolved oxygen as readily as cooler temperature water.  A shortage of oxygen can be noticed by observing the Koi in the early morning when levels would be lowest.  Koi are hovering at the surface of the water, gasping for air or if they appear to be lethargic are both signs Koi are suffering from oxygen deficiency.  For more on oxygen deficiency and helping control it, read this informative article.


Ammonia levels are an ever-changing entity in the pond world.  A few hours after feeding, ammonia levels will be higher due to the fish excrement versus the opposite reading you would get prior to feeding.   Interesting enough, it can also be noted that oxygen levels after feeding are lower due the fish metabolizing their food.

Ammonia is found in two forms in the pond: (NH3) and (NH4).   It’s fair to say that both of these form of ammonia are present in the pond at all times, one is just more prevalent than the other based on the current pH of the water.   Ammonia (NH3) changes to ammonium (NH4) as the pH level drops, and becomes less toxic.  As the pH rises naturally during the day, the ammonium converts back to toxic ammonia.  So, the toxicity of the ammonia is pH dependent.


It’s good to remember that while Koi have evolved into fairly resilient creatures, helping them to prevent stresses that could weaken their immune system and leave them vulnerable for diseases are what we all strive for.  Understanding these individual entities and how they interact with one another can help us to keep our fish and pond a healthy one!



Autumn Feeding of Your Koi


In many parts of the country, the transition into autumn brings cooler temperatures.  As pond water temperatures decrease, so does your koi’s nutritional needs.  Changing your koi’s food to a Spring & Autumn blend helps prepare them for the upcoming winter and helps to assure a great start for them next spring.

These cooler temperatures reduce your koi’s metabolism, which directly affects their ease of digestion.  High protein foods generally fed to fish during their active growing season of summer are difficult to digest as their metabolism slows.  Spring and autumn foods are specially formulated with less protein and more wheat germ, which is easier to digest.

Always follow the manufacturer’s recommendation on feeding amounts.  Excess food not eaten will result in poor water quality in the form of ammonia.  To top it off, these lower temperatures mean that our biological filter will not be operating at its peak efficiency to help break down this ammonia.

Aeromonas Alley, What Is It?

Fish living in cold-climate ponds have the added stress of dealing with a long winter of fasting, thus naturally weakening their immune systems. This, in turn, makes them more vulnerable to illnesses. It’s during those pond temperature ranges between 42°F – 62°F that are known as “Aeromonas Alley.”
To understand it better, let’s look at it a little deeper. After a long winter and as spring temperatures start to increase to around 55°F, our fish are slowly adapting to their environment and rebuilding their immunity. At the same time and at an even lower temperature just above 40°F, the Aeromonas bacteria become active and continue to increase exponentially with every degree of temperature. They already have a head start on our beloved fish’s immune system. Throw in the fact that the biological filter is only operating at 50% effectiveness at 60°F definitely puts the odds against the fish.
These Aeromonas bacteria are behind the name “Aeromonas Alley” and while most ponds show presence of this bacterium, as well as many others, a healthy pond has the proper balance needed to keep these guys in check.
While this condition can happen in the spring or the fall when the temperatures dip within the particular range, it is more common in the spring. Many experts believe it is more prevalent in the spring because the bacteria take advantage of the fish’s’ weakened immune systems vs. the fall when the fish are at their healthiest and strongest of the season.

Mouth Rot - Image Compliments of

Symptoms that your fish may suffer from the Aeromonas bacteria include: ulcers, fin and mouth rot
• Regular Water Testing for Ammonia and Carbonate Hardness (KH)- Most pond owners will ignore this activity in the winter but fish still release ammonia through their gills. In addition, the biological filter will be inactive so the ammonia will not be converted to nitrites at this point. If ammonia levels are elevated, partial water changes can help to control the situation.
• Carbonate Hardness testing is important because CO2 is still being produced, which is neutralized by the carbonates in the water. If the carbonates become exhausted, the CO2, which forms carbonic acid, will cause a pH crash. Some have found controlling the pH raise using baking soda. A more long term solution using oyster shells or crushed coral placed near flowing water has been successful for many.
• pH testing can be used as a backup. If there is a downward trend in pH, this is indication of insufficient carbonate.
• Keep filters clean to minimize the buildup of pathogenic bacteria.
• Minimize feeding of your fish in this temperature range.