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Why large weekly water changes are beneficial

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Earlier today a friend of mine asked me why I always do such large weekly water changes in my fish tanks.

When I tried to explain how it prevents the build up over time of toxins in the tank, he was having difficulty appreciating the difference that doing 50% makes over say for example 20%, so I decided to show him the numbers, and when he saw them he was a little surprised to say the least, and will be changing his water change regime considerably as a result.

Anyway, as I had all the figures, I decided to post them here, mainly so that I can point people to this thread any time I get asked why I feel such large water changes are desirable.

First of all, this is an illustration and doesn't relate specifically to any particular toxin, it just demonstrates how the water chemistry can change over time without you realising it.

For the sake of argument we'll assume that some toxin (it could be anything, hormones, pathogenic bacteria, organic waste, nitrates, etc.) is added at a rate of 100 units per week to the tank water. For the sake of argument we'll call it PPM, but it could be any unit of measurement and any amount.

Secondly we'll assume that the tap water contains zero units of this toxin.

Thirdly we'll assume that we are starting with a tank of new tap water with no toxins in it.

Now, if you do a 75% weekly water change (as I do) the following table shows how this toxin increases over time, and how quickly it reaches maximum concentration.

75% Weekly Water Change
Week: 01      PPM = 100
Week: 02      PPM = 125
Week: 03      PPM = 131
Week: 04      PPM = 133
Week: 05      PPM = 133

You can see that after 4 weeks, the toxin has reached the maximum that it ever can, based on the rate of introduction. It also shows that with a 75% weekly water change, it and for that matter any toxin, can only ever reach a concentration of 33% above it's weekly rate of introduction. Therefore the tank water will always have a very similar chemistry to the tap water.

The next table is for a 50% weekly water change. This time we can see that it takes 10 weeks to reach maximum concentration, and that it levels out at double the weekly rate of introduction. In general this should still maintain good water chemistry, except in the event that toxins are being added at a very fast rate throughout the week (i.e. in a very badly overstocked tank).

50% Weekly Water Change
Week: 01      PPM = 100
Week: 02      PPM = 150
Week: 03      PPM = 175
Week: 04      PPM = 188
Week: 05      PPM = 194
Week: 06      PPM = 197
Week: 07      PPM = 198
Week: 08      PPM = 199
Week: 09      PPM = 200
Week: 10      PPM = 200

Let's now compare this to a 20% weekly water change, which seems to be quite a common amount for people to do for their water change.

20% Weekly Water Change
Week: 01      PPM = 100
Week: 02      PPM = 180
Week: 03      PPM = 244
Week: 04      PPM = 295
Week: 05      PPM = 336
Week: 06      PPM = 369
Week: 07      PPM = 395
Week: 08      PPM = 416
Week: 09      PPM = 433
Week: 10      PPM = 446
Week: 11      PPM = 457
Week: 12      PPM = 466
Week: 13      PPM = 473
Week: 14      PPM = 478
Week: 15      PPM = 482
Week: 16      PPM = 486
Week: 17      PPM = 489
Week: 18      PPM = 491
Week: 19      PPM = 493
Week: 20      PPM = 494
Week: 21      PPM = 495
Week: 22      PPM = 496
Week: 23      PPM = 497
Week: 24      PPM = 498
Week: 25      PPM = 498
Week: 26      PPM = 498
Week: 27      PPM = 499
Week: 28      PPM = 499

As you can see even with weekly water changes the toxins continue to increase in concentration for 6 months, and they level out at a concentration 5 times the level of weekly introduction. To all you chasers of low nitrates, this means that if the nett amount of nitrates added per week (what the fish add, minus what the plants use, plus nitrates in tap water) is only 50ppm, then after 6 months your tank will never have less that 200PPM  and as much as 250PPM nitrates!

Obviously at this point all toxins will found in the tank in concentrations around 500% of the weekly rate of introduction, and the water chemistry is now considerably different from your tap water.

This could cause problems if you have to ever do a large water change for any reason, and it could also cause problems with the introduction of new fish, as these will likely be used to water that has the same chemistry as your tap water (If purchased from LFS).

Now let's look at things with a 10% water change, another fairly common amount given as a water change regime. (I have only listed the figures for alternate weeks to keep the list shorter, however the calculation was performed based on a weekly water change).

10% Weekly Water Change
Week: 01      PPM = 100
Week: 03      PPM = 271
Week: 05      PPM = 410
Week: 07      PPM = 522
Week: 09      PPM = 613
Week: 11      PPM = 686
Week: 13      PPM = 746
Week: 15      PPM = 794
Week: 17      PPM = 833
Week: 19      PPM = 865
Week: 21      PPM = 891
Week: 23      PPM = 911
Week: 25      PPM = 928
Week: 27      PPM = 942
Week: 29      PPM = 953
Week: 31      PPM = 962
Week: 33      PPM = 969
Week: 35      PPM = 975
Week: 37      PPM = 980
Week: 39      PPM = 984
Week: 41      PPM = 987
Week: 43      PPM = 989
Week: 45      PPM = 991
Week: 47      PPM = 993
Week: 49      PPM = 994
Week: 51      PPM = 995
Week: 53      PPM = 996
Week: 55      PPM = 997
Week: 57      PPM = 998
Week: 59      PPM = 998

As you can see, things really deteriorate now, and it's over a year before the tank reaches maximum concentration of toxins, and they are at levels around 1000% above the rate of weekly introduction.

In my eyes, this is nothing but a soup of organic horribleness, and not something I would want my fish living in. Also introducing any new fish to this must be a recipe for disaster, and any attempts at acclimatising them when you get them home are going to be fairly futile. Who gets the blame when the new fish die soon after they are added to the tank, the LFS possibly?

The last two examples quite clearly demonstrate why it's quite possible to run a tank for many months without any apparent problems and then, without making any changes to the weekly routine, things can start to go awry and the condition of the fishes starts to deteriorate as the levels of toxins increases.

Once this starts to happen, it's quite common that a large water change will be carried out, which due to the large difference in tank water chemistry compared to the tap water, can cause TDS shock and actually kill or badly stress the remaining fish. Ironically this leads to the belief that large water changes are bad for the fish!

So there we have it, that's why I always do (and recommend others do) at least 50% weekly water changes and preferably 75%, and why I think the fish will be much healthier as a consequence.

Sorry about the mega long post, but it wasn't possible to illustrate everything and keep it much shorter. If you made it this far without skipping bits then kudos to you :)

This is a great post. Should be Stickied!


--- Quote from: Sacha on January 22, 2015, 09:18:51 pm ---This is a great post. Should be Stickied!

--- End quote ---

Agreed! I wish LFSs would stop dispensing the never more than 50% rule... idiots!

Thanks guys, glad someone found it interesting :)

Does anyone here know what % water change a LFS would do? I doubt any of them do even close to 50%. Combined with high stocking and large evaporation area, every LFS I've ever been to has kept fish in elevated levels of everything.

While I don't disagree with the number theory in principle, the examples are harder to swallow. We can restate the long term steady level is approximately 2x, 5x and 10x the weekly addition rate for 50%, 20% and 10% weekly changes respectively, assuming we're not adding in anything with the change, nor is there any consumption in tank. Logically we would need to look at what the long term level actually is, and compare that to the safe limit. Only then could you say if there may be a long term problem or not. If we are adding something through changes, or have consumption in tank, that will skew the figures and increase complexity.

I wish freshwater tanks could be run more like marine tanks, where methods are available which can significantly reduce and in some cases even eliminate the need for routine water changes. I don't run those systems, which have other tradeoffs.


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