Adding Fuel to the Nitrate Debate: Why Water Changes DONT Reduce Nitrates

[JardiniBoy]

Taken from FishYou.com (it's not a board so I don't think it could be considered as MFK ... mods, please feel free to disagree)

The whole article can be found here:

http://www.fishyou.com/fish-nitrates-water.php

Just sharing info that I found interesting ... what are your thoughts?

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.

 

[shekes]

That is why large WC are better.

 

[meiling]

I am by no mean an expert or even close, but I have a set routine or water changes and water curing time that my aquarium always stays flatline on perfect params, but I do notice the smell after any change large or small. (You know, that fish tank cycling smell?)

 

[shekes]

 

[repair]

His thinking is flawed.

He isn't taking into concideration that when you remove 50% of the water you remove 50% of the nitrates and then you dilute the remaining nitrates.

The nitrates will continue the climb untill it stabilizes.

 

[shekes]

Your thinking is flawed.

"The waste in the tank on the evening (after a WC) of the first day = W(1-WC%)

Waste in the tank after second day (after WC) = W(1-WC%) + W(1-WC%)(1-WC%)

Waste on third evening = W(1-WC%) + W(1-WC%)(1-WC%) + W(1-WC%)(1-WC%)(1-WC%)

Etc., etc.


In English it means the fish are constantly putting out waste and it is being partially removed every evening with the water change, so the waste builds up over time. It is shaped like a learning curve where it is steep at first then gradually levels out to a nearly constant amount (the time to level out (flattening of the curve) varies with the WC% but is around one week for most of our situations). The actual curve if done by hours would actually form a sawtooth pattern but still in the general form of a learning curve."

For a more exact info click here

If you have difficulty understanding, you can do the following experiment:

Take a bottle of milk and perform small WCs on it. Try pouring out some 5% as some "small WC followers" suggest and refill with water. Shake and repeat. Shake and repeat. Shake and repeat. Shake and repeat. Shake and repeat. Shake and repeat. Shake and repeat. Shake and repeat. Shake and repeat. Shake and repeat.

You'll see that it will take you a whole damn day to get the water in the bottle absolutely clear. If you pour all the milk out at once and refill with water it will take a few seconds.

 

[meiling]

You all are going to have a pop quiz on this subject so get your facts together.

 

[shekes]

A quiz on flawed thinking?

Repair wins he is a moderator.

 

[HarleyK]

Howdy,

I think whoever posted the original also posted that his/her conclusion is wrong: By the end of week 4, he calculates 9.37 units. Without water changes, it would be 40 units. Now tell me how someone could possibly conclude that water changes do not reduce nitrates They're down to 1/4 !!!

The truth is, water changes cause pollutant levels to asymptotically reach acceptable steady state levels instead of continuing with linear increase.

HarleyK

 

[Jesse]

It's analogous to the frog that hops 1/2 way to the end of the log with each hop, never reaching the end because it's always hopping 1/2 of the remaining distance, however diminishing that distance may be. Well, we're not automatons who always do the same percentage water change at the same frequency. We're perfectly capable of measuring our nitrates and determining what percentage water change we'd like to make in order to reach the desired end nitrate concentration.