Why water change does not control nor eliminate nitrates

Too many people think that water change is an effective control of nitrates in aquarium. In reality, it does not and this article shows why.

The short reason why water change does not control nitrates is that you do not change 100% of your water. You only change a fraction of the total water volume, which means only a fraction of nitrates in the tank is removed everytime. If your fish always produce one unit of nitrate(be it daily,weekly or monthly) and you remove only a fraction of it (be it daily,weekly or monthly), the nitrates level will always be high.

If you didn't get that, here is an illustration

Let us imagine that 10 units (This unit is a unit of volume like mg, g etc and NOT ppm) of nitrates is produced in your tank/pond per week. The amount of nitrates produced is different for every tank/pond, depending on the number and size of fish, amount of feeding, rotting food/plants etc. You can be working with 10mg, 10g or even 10kg of nitrates but for this example we use 'units' as our unit of measure. For large fishes in ponds, you may be dealing with kilograms per week, in small tank maybe in hundreds of grams. For convenience we take our end product to be 10units.



Ok so if you are following so far, we established that our aquarium produces 10 units of nitrates per week. (We only use 'week' for convenience. If you subsitute with day or month the theory is the same.)

Now, suppose that we are trying to control nitrates using water change, and we change 50% of the aquarium water weekly (This is already a large water change, most people only change 20%-30%....


We start our calculation assuming we had zero nitrates initially..

Week 1

initial nitrates in aquarium = 0units

nitrates produced in one week = 10units

Total nitrates in aquarium = 10units

End of Week 1, 50% water change, resultant nitrates = 5 units

Week 2

initial nitrates in aquarium = 5 units (from the day before)

nitrates produced in one week = 10 units

Total nitrates in aquarium = 15 units

End of Week 2, 50% water change, resultant nitrates = 7.5 units

Week 3

initial nitrates in aquarium = 7.5 units (from the day before)

nitrates produced in one week = 10 units

Total nitrates in aquarium = 17.5 units

End of Week 3, 50% water change, resultant nitrates = 8.75 units

Week 4

initial nitrates in aquarium = 8.75 units (from the day before)

nitrates produced in one week = 10 units

Total nitrates in aquarium = 18.75 units

End of Week 4, 50% water change, resultant nitrates = 9.37 units


As you can see, as long as water is not changed out 100% (which most of us don't), nitrates will keep increasing as time goes by. Water changes does not control nitrate, but only slows down the rate of increase, normalising at the level that it is produced.



Why do some people think water changes can control nitrate?


1. Percentage value is not absolute value

Some people misinterprete the concept of part water change (or simple just didn't give it much thought.)

If you changed half your aquarium water for 2 times, have you changed 100% of the water?

If you changed one third of your aquarium water for 3 times, have you changed 100% of the water?

The answer is no for both although people have the tendency to think it is.


2. Confusion between nitrites and nitrates

Some people have no nitrate test kits and only use nitrite test kits. When they see their nitrites is zero, they assume their nitrates is also zero. This is totally wrong.


3. No abnormal behavior means under control

Some people don't measure nitrates at all. They look at their fish, everything appears normal, and they automatically assume everything is under control. The fact is that nitrates are not as toxic as nitrites to most aquatic animals, and if water changes have been done, it accumulates slowly instead of suddenly so there will be no obvious changes in behavior.


4. Time factor

If the tank has very low bioload and just not been used for long time, the concentration of nitrates is low because it has not accumulated for a long time. Also, if the water volume is very large but the bioload is small, it will take a longer for the increase in nitrates to be reflected using test kit, which measures in ppm, so it looks like it is holding stable.


To effectively control/eliminate nitrates, you need something that will consume them. This can be in the form of plants or a denitrator.



I received an email that basically says that this calculation is flawed and that water change can give a low ppm thereby keeping nitrates under control.
It is not. If you continue doing the calculations you will see that the readings will normalise at the nitrate production rate, which in the example given above, is 10 units. Now, this 10units is just an example and not a ppm value. If it was 10ppm, sure the nitrates is under control but it is NOT! If you want to peg a ppm value to it, do the following steps.
1. Measure the volume of your aquarium water.
2. Weigh the amount of food you put into the aquarium per week.
3. Divide the weight of the food by the volume of the water (remember to use the same units). if the result is 1/1000,000 that is 1ppm.
As an example, 45g in 450kg(4x2x2ft tank) is already 100ppm(And 45g of food is already next to nothing, most people feed way more. Again, this is an example only). If maintaining your aquarium at 100+ppm using water change means your nitrates is under control, good luck to your fish.

Lastly, this article does not condem water change. Water change is very much an essential part of fishkeeping as it renews the water coming into your aquarium system. Water change is also an important emergency action when your water is contaminated or has high levels of waste (even nitrates). What this article tries to show is how water change is not a long term solution to the problem. If your aquarium has no facilities that controls nitrates, sure, continue your water changes to "control" nitrate as keeping your nitrates at a high level is better than letting it rocket from high to higher.


Aquarium Information Home
Home > Advanced Fishkeeping > Why water change does not eliminiate nor control nitrates