even if you do 10% weekly water changes

Paradox of the partial water change…do you ever get it all?

I rolled my sleeves up tonight and did a water change. While I was elbow deep in tank-water I found myself wondering if it was possible to calculate exactly what the perfect water change frequency was. To keep things simple, I will just focus on a single parameter–nitrates. I know that we all do water changes for more reasons than just nitrate removal, but it seems like a reasonable place to start. My fish brain told me…that if I could figure out what my nitrate accumulation rate was…I could calculate the ideal water change schedule. What I found out was that it was harder than I had thought.

aquarium partial water changesLet’s assume, my tank accumulates, 5 ppm of nitrates that doesn’t get skimmed out each week and that I do the old textbook 10% water change weekly and I never miss a water change (hopefully my fish aren’t reading this article…or at least haven’t figured out how to use the comments section).

After 1 week, my nitrates should be 4.5 ppm (5 accumulated, removed .5) After week 2, the nitrates would be at 8.55 ppm (4.5 + 5 -0.95).¬†After week 3, the nitrates would be at 12.2 ppm. So clearly the 10% partial water change isn’t enough to keep up with things. Nitrate is building up and getting out of control.

So, my fish brain said, do a bigger partial water change… ok fish brain…let’s double it to 20%. Now the theoretical results are:
week 1- 4.0
week 2- 7.2
week 3-  9.8

The problem isn’t as bad…but it’s still clearly a problem. Now in all honesty, my fish brain started to catch on, and I imagine you are too–but just to make a point, let’s assume my fish brain told me to go even farther–assuming 50% water changes weekly.

Even in this extreme situation, the theoretical results would be as follows:
week 1- 2.5
week 2- 3.75
week 3- 4.375

The math goes on and on like that and gets stuck just below 5 ppm.But let’s face it, THAT is a lot of water to change every week. Who has the time, energy or money to do that? And I was intrigued by the fact that the concentration got stuck at 5ppm. So I went back to my earlier calculations–and if I hypothetically did a 10% water change–my nitrates would steadily rise until they got to a whopping 45 ppm and then basically stop. So then, exhausted by the math, I did one final ‘in between’ value–at weekly 25% water changes, the nitrates would plateau at 15 ppm.

even if you do 10% weekly water changes

Now my wacky math needs a lot of disclaimers. For one, I arbitrarily picked 5ppm as the accumulation rate. If the accumulation rate was just 2 ppm, my nitrates would plateau at 2, 6 and 18 ppm with either 50%, 25% or 10% weekly water changes.

The reason I bring you through all this math is for the following simple message…don’t just take an old rule of thumb like…do a 10% weekly water change…as gospel. If you can figure out your accumulation rate, you can do a calculation to see where your own tank is going to get stuck. If you’re trying to figure it out for yourself…don’t deal with all the math I did, just take this shortcut:

  • If you do 50% water changes forever, your water parameters get stuck at exactly 1 x’s the accumulation rate.
  • If you do 25% water changes, your water gets stuck at 3 x’s the accumulation rate.
  • Finally, if you do 10% water changes, your water gets stuck at 9 x’s the accumulation rate.

even 25% water chagnes

For me, the other take home message is that I’m going to do a major water change–with purpose–on a periodic basis to help remove whatever is building up in the tank that I don’t even know about…

So what do you think…am I crazy for doing all this math?


  1. Doing only water changes is not the answer.
    Partial water changes and carbon dosing will definitely help to control nitrate and phosphate build-up in the long run.

  2. Author


    Thanks for the comment. You are very right. Partial water changes and carbon dosing will help! Thanks for the note. How long have you had your tank?

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