We own a three thousand-gallon pond and every month we do a 10% water change. We have talked to other hobbyists and some say they do a water change and others say they do not change any of the pond water. My question is: When changing water in a pond how much and how often should it be changed, if at all?

Q:

We own a three thousand-gallon pond and every month we do a 10% water change. We have talked to other hobbyists and some say they do a water change and others say they do not change any of the pond water. My question is: When changing water in a pond how much and how often should it be changed, if at all?

A:

Nature’s waters are abundant in biological materials, ranging from microscopic organisms too large aquatic plants and animals including fish. The presence of plants and animals in the aquatic environment means that there are also organic and inorganic byproducts being mineralize from solid organic materials from living or dead tissue. These breakdown products include humic acid, oils, waxes, assorted hydrocarbons, and fatty acids “all invisible residues that affect water quality. Although, it does not matter whether it is a lake, river, or the vast oceans, the waste generated by fish, our aquatic animals and plants do not accumulate to any significant extent the sheer volume of water of the habitat is diluting it. The pollution concentrations are also eradicated and diluted largely in cases such as rivers and natural pond waters, since freshwater is renewing it constantly through the intersection of topography, being stream-fed with freshwater, rainwater, and meltwater from ice or snow. 

However, the typical ornamental pond operates as a closed recirculation system, with the same water remaining in the pond for weeks or months at a time, even if it rains frequently this will make an insignificant difference. In this situation water quality is always a problem because as time passes various physical, chemical, and biological processes working in and around the pond alters the initial tabula rasa characteristics of the water. 

 In an enclosed ecosystem such as our ponds, a void of an overabundance of plants and ion nutrient users, most hobbyists think they can make their ponds oligotrophic in nature, but this is much harder than one thinks to achieve. Because; most of the time filtration systems are inadequate at the removal of pollutants generated by the inhabitants and clean highly oxygenated water is dependent upon the filtration systems capabilities and the amount of water that is being exchanged by the hobbyists periodically. 

 There is little argument whether a periodic partial water change is necessary to maintain a healthy pond that fish can live in without undue stress. I also think that all hobbyists would agree that all ponds would benefit from more frequent water changes and generally this would be “the more frequent the better.” However, how much water should be renewed and how often should such changes take place are often a matter of discrepancy. Finding unambiguous answers to these questions in hobbyists’ books and monthly periodicals may become a crapshoot at best. Too many hobbyists do not understand the mathematical equations used to determine whether water changes would become beneficial or redundant in an enclosed ecosystem such as our ponds. 

One thing hobbyists must understand is the idiom Pollutant Equilibrium (or PE for short). Pollutant Equilibrium means that the amount of pollutants that are being produced by the animals, plants, filter and the amount the water that is exchanged from periodic water changes will reach what is called a steady state or constant state. This means when a steady income of pollutants are being produced at a given rate and water is being exchanged at a given rate that everything will remain on an equilibrium with each other and nothing will increase or decrease over a given time. If pollutants overshadow the amount of water being exchanged then the amount of pollutants will increase over time to toxic levels even though a constant amount of water is being replaced. This arises due to inadequate filtration systems the hobbyist thought would work for their fish load. 

For example; let’s say you have a pond,  for the sake of argument will say this pond is 3000 gallons, that is producing 8-ppm (ppm = parts per million) of nitrogen (NO3) every month, this now becomes a constant. The hobbyist now wishes to reduce this nitrogen compound by doing a water change on a monthly basis. If the hobbyists were to do a 50 percent water change, this now would halve the amount of pollutants to 4-ppm (0+8)-50%= 4). However, do not forget that every month the NO3 levels will begin again to elevate another 8-ppm. In addition, you must include the NO3 compounds that were remaining from the last water change. The next month will make the pollutant level elevate to 12-ppm before a water change (4+8)-50%=6). The next month after that it will elevate to 14-ppm (6+8)-50%=7) and so on in their pond. It would now take eight months before a PE is then reached.

 

Then every month afterwards, the Nitrogen compounds being produced, and the amount of water being exchange would be in equilibrium with each other, and would remain at a constant 15.9-ppm NO3 levels or a steady state. As you can see the hobbyists even after conducting a 50 percent water change of 1500-gallons, may still run into problems with cyanobacteria and algae buildup, as green water in the pond. Because their nitrogen compounds have now exceeded the safety margin of keeping nitrates below the 15-ppm limit every month before a water change is executed. 

 If the same hobbyists were only to do 20% water change every month, it would take over sixteen months before a PE would be reached of 39.0-ppm NO3 levels. If the water changes were only 10%, calculation similar to those used above, would show the ensuring situation deteriorating even further, with the pollutants stabilizing at 20 times the amount generated from one water change to the next. Besides, the PE values differing with different extent of water being change the time it takes for PE to be reached also differs. In reality, doing a water change of anything less than 40 percent would be useless in anybody’s pond. The consequences of increasing or decreasing the frequency of water changes or the volume of water replaced on each occasion would only be anyone’s guess. Do to the fact the hobbyists not knowing the exact aleatory nature of the biomass and how much pollutant matter is being generated in a single day of the pond existence. 

Therefore, with the information that we now know regarding the buildup of pollutants and routine partial water changes we can conclude the following. Starting with pure unpolluted water, pollutants in the pond will progressively increase with time, even as partial water changes continue at regular intervals. However, this increased does not continue unabated but stabilize as it reaches PE. The greater the proportion and the shorter the régime-time between each renew water change the lower the PE would be and the shorter the time it will take the PE goal to be reached. As you can see, winning the battle against pollutants in an enclosed biotope such as our ponds seems almost to be futile1. This is because the amount of nitrogenous wastes produced is many times greater than the pond’s natural capacity to absorb it.

 However, one of the biggest weapons we have in our arsenal which is in our favor is a well-designed filtration system, which we can implement in the battle against the pollutants. Now we come down to the one big problem that all hobbyists are faced with and that is “the well-designed filtration system.” Since this is easier said than done the hobbyists are left with no other alternative than to do partial water changes in their pond.  

Even with the Anoxic Filtration System, as good as it is still needs to have at least two partial water changes made each year. Generally, the greater proportion of water that is changed during the filter cleanout the lower the stabilizing pollutant level in the pond would be. Because of this filtration systems capability, the Pollutant Equilibrium levels are reached within a short time-span of weeks instead of months, without all the frustrating water changes and the cost of doing them. From what we have learned, the hobbyist that does 10 percent water changes would hardly be worth the endeavor or their valuable time.

 

1: The fact is that the actual ammonia, nitrite, and nitrate levels in a fully recirculating biotope such as our ponds that requires supplementary biological filtration is never zero “even if the filter design ensures 100 percent inorganic compound removal effectiveness. There is always some trace amount of these compounds in bulk water because the fish are constantly adding ammonia (fish continuously excrete ammonia through their gills, as well as through diluted urine) and other organic compounds to the water body proper. The filter can only remove ammonia and nitrites from that small portion of the pond water that is moving through it at any given time. So, even as one portion of the pond water is being cleansed of these compounds, another part is being polluted at the same time.