DoctorJ's Blog

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Toxic waste cocktailCreative Commons License courtesy of P a U L i u S

So, how many water changes does it take to completely replace ALL of the water in your tank?

WHY would you want to do this? Over time, bad things like trace metals and other toxins that your cycle doesn’t remove build up in your tank. They can become concentrated due to evaporation, and maybe some other processes. Check out Snowman’s awesome blog post on Old Tank Syndrome here: and learn more.

But how many PWCs does it take to replace all of your old water? You might be wondering just as matter of curiosity, or have some other ideas. But, I must tell you that doing four, 25% partial water changes does NOT replace all of your water. Not even close.


Imagine you have a glass of milk. You dump out 25% of the milk and add water. Now you have a full glass of watered down milk.

Do it again.

You still have watered down milk. Just more watered down.

Do it again.

Even more watered down milk.

In fact, it would take you an extraordinary number of milk dumpings to get rid of that last molecule of milk to give you pure water.

Now imagine milk was ‘old water’ and realize that even partial water changes aren’t ever going to replace all of the water in your tank. In fact the only way to do that is to replace ALL of your old water in one water change. I think most people here would recommend against that. It stresses your fish out.

I did work up a formula for this, but it is a little complicated, so please allow me to give you tables you can use for 25% and 50% water changes. For the purposes of this blog, I’m going to estimate that your fish are ‘safe’ if you’ve replaced 95% or better of their water.

Please note that these tables are independent of your tank’s size because they are percentages.

* 50% Partial water changes *

Note that each water changes replaces ‘half’ of the old water remaining.

Start: 100% old water 0% Fresh water

1st 50% PWC: 50% old water 50% Fresh Water

2nd 50% PWC: 25% old water 75% Fresh Water

3rd 50% PWC: 12.5% old water 82.5% Fresh

4th 50% PWC: 6.25% old water 93.75% Fresh

5th 50% PWC: 3.13% old water 96.87% Fresh

So, it takes five, 50% PWCs to replace almost 97% of your tank’s old water. You could do these one a day, or maybe two a day as Snowman suggests in his blog (linked above).

* 25% Partial water changes *

Note that each water change replaces a quarter of the old water remaining

Start: 100% old water 0% Fresh water

1st 25% PWC: 75% old water 25% Fresh Water

2nd 25% PWC: 56.25% old water 43.75% Fresh Water

3rd 25% PWC: 42.19% old water 57.80% Fresh

4th 25% PWC: 31.64% old water 68.36% Fresh

5th 25% PWC: 23.73% old water 76.27% Fresh

6th 25% PWC: 17.8% old water 82.20 % Fresh

7th 25% PWC: 13.35 % old water 86.65 % Fresh

8th 25% PWC: 10.01% old 89.99 % Fresh

9th 25% PWC: 7.51% old 92.49% Fresh

10th 25% PWC: 5.63% old 94.37% Fresh

11th 25% PWC: 4.22% old 95.78% Fresh

As you can see, it takes a lot more work to replace the majority of your old tank water using 25% PWCs, although it is much gentler on your fish.

What if you only wanted to do one PWC per day for one week? You don’t want to start with a 50% PWC, as that changes your water parameters too much. Doing Seven PWCs, with 25% the first four times and 50% the last three times gets you there. It also limits the amount of change your fish experience (the first partial water change will expose your fish to 25% new water, while the first 50% change on the fifth day changes it by just under 16%). You might try this if you want to get it done in a week. (You can’t really rush this any more and do it safely. For example, you could do a 50% PWC on day four, but you still have to do a 50% PWC on the next three days to reach the goal. Why put your fish through it if you don’t need to?)

* The one week plan *

Start: 100% old water 0% Fresh water

1st 25% PWC: 75% old water 25% Fresh Water

2nd 25% PWC: 56.25% old water 43.75% Fresh Water

3rd 25% PWC: 42.19% old water 57.81% Fresh

4th 25% PWC: 31.64% old water 68.36% Fresh

5th 50% PWC: 15.82% old water 84.18% Fresh

6th 50% PWC: 7.91% old water 92.09 % Fresh

7th 50% PWC: 3.96% old water 96.04 % Fresh

I hope someone finds this helpful, and give me a shout out if you do. 

So... what do you think? Please leave me a comment or give me a


  • Vale: Great blog! This should help people understand the water changes better! By the way - it kind of reminds me of those math problem solving questions :)
  • Mmsculpture: This is great. I just did this and guessed pretty correctly. LOL. But I couldn’t figure out the actual math :) Thanks!!
  • EvB: This is very good info, and I like the way you quantified Snowman’s concept here.
  • briantheref7: A very good read Doctor, and very interesting.
  • Tropics: Thanks DoctorJ,
    Very useful info, I love math, numbers and tables and it will help me to keep track on how much fresh added and how much fresh still need to be done, to be sure I changed all old water. That’s really awesome. Snowman’s blog is very interesting and educational, yours are a precise continuation! Great job.
  • Dex07: I found this extremely interesting as I am in the process of decreasing the concentration of salt in my tank (was treating fin rot). I want to do it gradually so as not to stress my Betta. Have been doing a 25% PWC twice a week.
    This gives me a good idea of how much I need to exchange.

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