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This website is dedicated to Ol'Conrad.  He passed away shortly after Christmas, 2013.  He had made stupendous contributions to our hobby through his dedicated and careful breeding of aquatic animals.  He was a good friend and best buddy of Pete's and will not only be missed greatly by Pete, the hobby will forever have lost a valuable friend and asset. But his progeny will live on forever in his memory.
The Solution to Pollution is Dilution, at least for fish.

  If you do not like math, skip to the last sentence. You are warned. And remember the last time I warned about something... it wasn't pretty.

  A few years ago Fishguy Paul did a spreadsheet on waterchanges and nitrates but I haven't been able to find it. Something to the effect of if you had the fish contributing nitrates each week, and you always did the same frequency and percentage of waterchange, at some point it would get to a certain level and hold fairly steady (maybe at a level higher than one would expect).

  I don't care about nitrates exclusively, for me it's more about general dilution of waste and pollution. I used nitrates because it can be measured and is both familiar and universal.

  The size of the tank doesn't matter & the water supply has 0 nitrates.
  The starting level of nitrates is 50ppm and the fish waste adds 20ppm nitrates per week.
  The fishkeeper will do a 25% waterchange each week.

  After that first change, the level of nitrates drops to 37.5ppm, over the course of the week the fish eat and excrete adding 20ppm nitrates. Now, after a week, the tank has 57.5pmm nitrates (up 7.5ppm).

  The fishkeeper does another 25% waterchange, the levels drop to 43ppm, fish add 20ppm during the week and it ends at 63ppm nitrates (up 5.5ppm).

  The next 25% WC drops it to 47, fish add 20ppm, now it's 67ppm (up 4ppm).

  Each week the level of nitrates goes up a little, but as each week passes the increase (over the week before) is slightly less as it gets closer to equilibrium. After about 3 months it levels out and at 6 months stops climbing completely. At about 80ppm, the fishkeeper is taking out an equal amount of nitrates as the fish are producing.
If the fishkeeper does a 40% water change instead of 25%, the nitrate levels will never go over 50ppm.
If the fishkeeper does a 50% WC, this is what happens. Nitrates quickly settle down to no more than 40ppm.
This is 60%
And 75%
My point is that big waterchanges do a better job of removing pollution and greatly dilute what remains. Fish produce more than just ammonia (end result is nitrates), other things accumulate in the water too, but nitrate easy to measure. You can also use this to estimate the minimum waterchange you should do. Measure the nitrates right after you do a waterchange, that's "b". Measure it again right before you do the next one and the difference is "e".

b = beginning nitrate level
c = percentage of water change
e = nitrate fish will add
f = end nitrate level
b * (1 - c/100) + e = f

If f is bigger than b, do a larger waterchange.

OR, you can just skip all this headache inducing math crap and do a big waterchange. 

If you have been doing small waterchanges for months or years, the water in the tank has a ton of accumulated stuff dissolved in the water (TDS - total dissolved solids). My OPINION is that suddenly doing a large waterchange can really shock the fish and potentially cause them serious problems with osmotic regulation. My advice would be to slowly increase the percentage of water being changed and/or the frequency. As the tank water becomes closer to the tap water, the size of the waterchanges can increase.

This is also why just topping off evaporated water but not doing water changes isn't a good idea. You end up with water that is completely different from your tap water.

Same with reducing salt levels after treating for Ich (or whatever). If you have 3 tablespoons of salt per gallon, you can't just do a 75% water change to get rid of the salt without possibly screwing up the fish pretty badly.

Angels Plus has a decent article on this. If you can make it through the first few paragraph it starts to make more sense.


Pete has been pushing this for years.  NOW you have Beth pushing it too!  Don't you late bloomers or newbys realize they are not talking cause they love to hear their own voice??

And if you go check out Aquaboards.com you will find a chorus of echos saying the same thing.

​Thanks Beth,i pay for water here and was curious as to what my minimum waterchange would be. Number of fish increasing or decreasing and the variable of feeding would be a factor,i imagine. question from: dsuperman

Just a question, would the values change if vegetation was factored in, meaning the nitrates and other tds that they would use up. Or would the values only change say, for a few weeks maybe after they are added to the system?  question from:  metalShrek
Everything is a variable so there is no one-size-fits-all amount of water which should be changed. When I am not able to do my usual waterchange routine, I feed them less and make sure the tank is covered so the water won't be evaporating.

Plants do not use everything which may be in the water and of what they do use, they don't necessarily use it in the same proportions that may be in your water. If you are only concerned with nitrates, plants will lower them.

In the summer I have 55G tanks outside and the nitrates are always zero because of the plants. But I still do waterchanges because the water is evaporating and to get rid of anything else that might be building up.

Waterchanges can also replenish good things which might be used up by plants or biological processes (carbonate, calcium, magnesium etc).

I used nitrates as an example but I never test for nitrates. I do waterchanges partly because I like doing them and partly because the fish like when I do them. My fish ALWAYS look better when they get regular and significant waterchanges. 
That is exactly right. I don't even own a nitrate test kit.

You can test for lots of things to get a general idea of your water quality. Here in NY our water comes out of the tap at about 7.4-7.6 PH. When folks would walk into the old fish building with a water sample if their PH was testing low their hardness would be through the roof along with their nitrates.

It was always the same story. They were not changing enough water to dilute it enough to prevent "stuff" from building up in their tanks. If acids were building up so was everything else.

That's why it's moronic to ADD stuff to your water to make it "better". The idea is to NEVER add stuff, always dilute stuff back OUT of your water.

If someone has so much acid building up in their tank it has moved from 7.6 down to 6.8 that means EVERYTHING else had built up in their water too. By adding a ph increaser they MAY get a temporary boast in PH but that won't reduce their hardness, lower their nitrates OR dilute all the rest of the CRAP that is slowly building up in their tanks.

I enjoyed reading your charts very much Beth but that last line is what is most relevant.

A person can have a tank JAMMED full of plants and a zero nitrate reading but it ONLY means they have lots of plants and no nitrate build up. It doesn't address ALL the other buildups they do have.

One of my biggest beefs going in this hobby is ALL the newer products like Tetra "Easy Balance" or ANYTHING that claims it eliminates water changes. NONE OF THEM actually work and NOBODY outta be using or recommending these products. EVER!

You just plain old can't dilute water by adding stuff to it. That goes against all logic folks. It's like adding something to a bucket of water to make it weigh less. It's not going to happen.

Any more questions?   
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I do about a 75% water change every other day on every tank I own and I don't test for anything. I have had people tell me that I am changing too much water too frequently but for me it sure beats the heck out of worrying about nitrates and other harmful stuff in the water. Call me crazy too but my fish swim around all excited looking when I am adding fresh clean water to their tank almost like they are giving me their approval. That last line makes it sound like I may need to cut down on my drinking in the fish room but I swear it's true.  contributed by - bubbydenaquatics

I have done a lot of testing over the years on the buildup and removal of dissolved solids in a tank. there are ways to completely remove almost everything without doing any water changes, but they arent easy and they require more initial volume and varied flora than most are ready to maintain. that said, the one thing i have had the most difficulty removing from water is sodium chloride, regular table salt. there are very few things that will remove it. the things that will either grow way to slow to be of any value, or require such a high salinity as to make the whole effort moot.

most of the time you can find a way to either sequester various compounds and minerals into forms that can be removed from the water column, but an easy way to remove salt still eludes me. for that reason, water changes are still an eventual necessity. contributed by - Auban