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Koi Pond Water Quality

by Elmer Epistola

Posted: October 13, 2004



Koi enthusiasts never take their pond water for granted.  Just count the number of articles on the web discussing how to keep your water clean enough for your koi.  There are literally hundreds of them.  Now add one more to the list -> this article.  I guess a koi website won't be complete without one.


Water quality, as said many times before and to which we all agree, is the most important factor in the pond that affects koi health (and happiness).  The question, therefore, is not on the importance of diligently keeping our pond water clean, but on how well we know if we're meeting the requirements.  Unless we have a way of measuring the cleanliness of our pond water, we can not really claim that we have good water, regardless of how frequently the mats are cleaned or how clear the water is.  Water testing kits, which are now widely available in the market, should therefore be part of any koi hobbyist's toolbox.


Koi experts agree that water is good enough for your koi if:  1) it is free of chlorine and other chemicals such as pesticides, heavy metals, organophosphates, etc.; 2) it has undetectable levels of ammonia and nitrite; 3) its hardness, pH level, and temperature are correct; 4) it has low levels of dissolved organic compounds (DOC) and particulate organic compounds (POC); and 5) it is stable in its quality.



Figure 1. Examples of water testing kits

for different water quality parameters



In the place where we live south of Manila, chlorine in the water is not an issue because we get our water from deep wells.  If you get your water from the tap, then chances are that it has been treated with chlorine.  You can actually 'smell' chlorine in the water, especially if you're like me who uses deep well water.  Chlorine, even at the level present in tap water,  is deadly to koi.  Thus, you must never put koi in pond just filled with chlorinated water.  'Aging' the water by 24 hours prior to putting your koi in is one advice that you can follow.  Later additions of tap water may be done as long as they're small in quantity compared to the bulk of your pond water.  Testing of chlorine level after each addition is highly recommended.


Ammonia (NH3) is the next deadliest contaminant in pond water, next only to chlorine.  One part per million is already detrimental to the fish.   The main source of ammonia, unfortunately, can not be removed from the pond.  This is because ammonia in the pond comes primarily from the koi themselves.  Ammonia will therefore always be present in the pond as long as there are koi (or other fishes and animals) living in it.  The good news is that ammonia is easily converted to less harmful compounds by certain types of bacteria (nitrosomonas) through biological filtration.  


The threat of ammonia contamination poses the highest risk while the pond is still new.  This is because new ponds still lack the colony of 'good' bacteria needed to convert ammonia into less harmful compounds.  Koi experts recommend daily checking of ammonia levels and daily 50% water changes in new ponds until ammonia readings stabilize.  The threat of ammonia in new ponds is so serious that there's even a name for this phenomenon - the 'New Pond Syndrome', or NPS.


I myself had been a victim of NPS when I was starting out.  At that time, I was mainly concerned with the adequacy of oxygen supply in the pond.  I just switched from aquarium keeping to pond keeping, and never even had the inkling that ponds are also filtered.   I enjoyed adding fish to the pond during the first week, only to be greeted by a couple of dead fish one morning during the following week.  The next few days that followed were horrific - fish died daily until I practically had nothing left.  In panic, I bought books about setting up ponds and immediately saw where I could have done things differently.  Novice hobbyists who are about to set up a new pond must not take this phenomenon for granted. 


At low levels (below 0.1 mg/liter), ammonia acts as a strong irritant to the koi, especially to its gills.  Flashing may be exhibited by koi irritated by low levels of ammonia. Higher ammonia levels can lead to skin and gill hyperplasia.  Gill hyperplasia refers to the condition wherein the secondary gill lamellae become swollen, resulting in breathing problems for the koi.  Serious gill disease and death can occur if the lethal levels of ammonia contamination are reached.


The acceptable ammonia reading for koi ponds is, well, zero.  Thus, your pond is threatened by ammonia as long as your test kits are able to detect ammonia in your water.


Sooner or later, the rate of ammonia level build-up in a new pond will decrease as biological filtration does its job, but another toxic compound will be taking over as this happens.  Ammonia is converted by the nitrosomonas bacteria into nitrites, which are also harmful to the fish, but not as deadly as ammonia.  Nitrite levels therefore shoot up as the ammonia levels go down in a new pond.


Nitrites (NO2) at low levels can subject the koi to stress, making it vulnerable to diseases caused by other factors.  High levels of nitrites cause skin and gill epithelia damage, which can lead to parasitic invasion or secondary bacterial infections.  Fish suffering from nitrite poisoning will be gasping at water surface and stay around water outlets.   Water testing kits must register nitrite readings of zero as well if the water is to be considered nitrite-safe.  Unsafe nitrite readings can be corrected by sufficient partial water change. Adding salt to the pond (around 0.02%) also makes nitrites less toxic to koi, since their gills will tend to take up the added chloride ions instead of the nitrite ions, getting protection from the latter.



Figure 2. An electronic device capable of

testing several water parameters



Eventually nitrite readings in a new pond will go down, just like the reduction of ammonia levels before it.  Again, the reason for this is the conversion of nitrites into nitrates by 'good' bacteria, this time the nitrobacter sp.  Nitrates are significantly less toxic than nitrites, but they should still be included in regular water quality checks.


Nitrates (NO3-) are generally harmless to koi, although koi had been observed to lose their appetite if the nitrate level is allowed to go unchecked.  Nitrate level should always be kept under 60 parts per million (ppm), which is equivalent to 60 mg per liter, although some koi experts say that a nitrate level of 100-500 ppm is not yet detrimental to koi.  Partial water change is a good remedy to higher-than-normal nitrate level.  The use of plants and trickle filters will go a long way in stabilizing the nitrate level of the pond.


A weekly test for ammonia and nitrate levels is recommended, although this frequency may be diminished if the pond has matured enough and stable readings are already being taken consistently.  Nitrate level testing can be done only once a month.  Of course, any change in the filtration system or koi pond itself necessitates a more frequent regular monitoring once more until the readings stabilize again.


Copper and iron are examples of metals that are toxic to koi.  Be sure that your pond water is not exposed to these metals.  Possible sources of copper and iron are your piping system, heaters, and even your filters.


Koi pond water pH is another water quality parameter that needs to be checked regularly.  The term 'pH' stands for 'pondus hydrogenii', and is a measure for how acidic or basic your pond water is.  A pH of 7 means that the water is neutral.  As the water becomes more acidic, the pH number goes down.  The pH number goes up as the water becomes more basic.  Koi pond water pH must be maintained between 7 and 8.5


Off-scale pH readings can result in direct physical damage to the skin, gills, and eyes of the koi. Prolonged exposure to incorrect pH can lead to stress and in extreme cases, epithelial hyperplasia (swelling of the gills) or even death.



Figure 3.  An off-the-shelf pH Test Kit



Water hardness, on the other hand, measures the mineral (calcium, magnesium, iron, zinc, etc.) content of the water.  These minerals are usually brought into the pond by rainwater that has percolated through rocks.  There are two hardness numbers usually monitored in koi ponds, i.e., the KH (for carbonate hardness) and the GH (for general hardness). 


The pond's KH level (also known as the alkalinity) determines its pH buffering capability, or the ability of a pond to keep itself from becoming too acidic.  The pH of a pond is affected by a lot of things (such as plant photosynthesis and the production of acids during nitrification), and can swing in value from time to time.  One way to stabilize the pH of a pond is by neutralizing any acids introduced into the pond with carbonate and bicarbonate ions, a process known as buffering.


GH is just the sum of KH and another hardness parameter, the permanent hardness.  In a koi pond, the permanent hardness is negligible compared to the KH, so GH and KH may be treated as one for practical purposes.


In most water systems, water hardness (GH or KH) is quantified in terms of the amount of calcium carbonate (CaCO3) in the water (mg per liter or ppm).  Very hard water contains greater than 300 mg of calcium carbonate per liter of water while soft water contains less than 75 mg per liter.  Koi ponds, according to koi experts, should have a water hardness of 100-150 mg CaCO3 per liter of water, or 100-150 ppm


Soft pond water should be avoided because it forces the koi to exert more effort in osmoregulation, the process of maintaining internal body water concentration to the correct level.  Furthermore, a low KH results in pH instability, which can be lethal in extreme cases of pH swing.  On the other hand, mineral deposits will form in the pond if the hardness exceeds 150 ppm. Note that some koi experts advise higher hardness levels than this (even up to 300 mg), if only to assist the fish in osmoregulation. 


Water to us is but something to drink, take a bath with, or swim in.  To our koi, however, it is what they breathe and live in, what dictates their bodily functions and, ultimately, what determines whether they will live or die.  Indeed, koi pond water quality is something that every koi enthusiast must never take for granted.





References: 1) www.fishdoc.co.uk; 2) http://www.geocities.com/koifla/New_Pond.htm;

3) Water Testing and Test Kits by Dick Roemer; 4) Koi World 2003-2004 Annual






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