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#### ray-the-pilot

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Which is better 4 x 20% water changes or 1 x 50% water changes
I didn’t think this was such a difficult concept but I guess it is?

First off let’s agree on what is better.
If I do a 20% water change, the resulting water is:
(1-.2) = .8 or 80 % of the original water is left.
If I do a 50% water change, the resulting water is:
(1-.5) = .5 or 50% of the original water is left

I am assuming we all agree that 50% changes more water than 20%?
Please note: The lower the resulting number in this calculation (which is the amount of the original water left) the more water is changed.

Now back to the question:
4 x 20% changes
(1-.2)*(1 -.2)*(1 -.2)*(1-.2) = .4096
(Only 41% of the original water is left)
1 x 50% changes
(1-.5)^1 = .5
50% of the original water is left.

So 4 x 20% changes more water than 1 x 50%

In general to find out how much of the original water is left, use the following formula:

(1 – A)*(1 - B)* (1 – C) * …
Where A, B, C, … is the % water changed divided by 100.

If you always change the same amount of water then the expression simplifies to:
(1 – A)^N
Where A is the % water changed divided by 100.
And N is the number of changes.

Which plan is more work is a relative thing.
If you have a 10 gal tank changing 50% means only changing 5 gal of water. Making, conditioning, adjusting and handling 5 gal is manageable for most people.
If you have a 100 gal tank, changing 50% means changing 50 gal of water. Making, conditioning, adjusting and handling 50 gal is going to require a lot more concentrated effort and even specialized equipment. You probably would want to do more frequent smaller changes.

Changing water is the best thing you can do for your fish/plants. It is also the riskiest and the more water you change at one time, the greater is the chance of making a fatal mistake.
For example:
Say your make up water is 22°C and your tank water is 32°C. If you do a 50% water change the new tank temp is:
.5 * 22°C + .5 * 32°C = 27°C (A change of 5°C)
A 10% change results in:
.1 * 22°C + .9 * 32°C = 31°C (A change of only 1°C)
So be very careful changing water!

#### jdm68

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For the reasons you outlined in your warning at the end of your post, I would go with more frequent, smaller changes. It also has the benefit of being more managable with smaller amounts of water. I do 5 gal changes on my 45g about 2x a week. Maybe I should do more, but params are stable and fish seem happy, so I'll stick with it until something changes...

#### Dempsey

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That's a little too much math for most of us even to want to try to figure out! :hihi:

The numbers are nice to see though.

In any case, I do feel that more and smaller water changes are great! Plants actually love it! In my tanks anyway.

I actually do 80% weekly water changes on most of tanks though. Still awesome results! This works well for what I am trying to do. 75g tank.... I would rather just do one water change a week at 80%. Never lost a fish or plant doing so. If I did(can't think of a time)it would be negligence on my part. Not adding Prime, not looking at the temp of the water that I am adding....

It's more of a matter of taste and goals.

With some of my smaller, low light tanks, I will actually do 2-3 20-30% WC's per week. Still dosing EI the whole week. Now, I don't dose the whole amount on some because I have played with the tanks and seen where I can lower this or that. Others, I dose full EI even with low light tanks.

Over kill? I'm sure it is.... That's fine with me though. Guess what I don't have to do? Worry about ferts, PPM of this or that.... They are cheap and I have pounds of them. Years and years worth even with the amounts that I dose.

It is really just a matter of what you want to do. How much work do you actually want to do...

Things can be much easier for folks if they just stop and think.....

roud:

#### plantbrain

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Which is better 4 x 20% water changes or 1 x 50% water changes
I didn’t think this was such a difficult concept but I guess it is?

First off let’s agree on what is better.
If I do a 20% water change, the resulting water is:
(1-.2) = .8 or 80 % of the original water is left.
If I do a 50% water change, the resulting water is:
(1-.5) = .5 or 50% of the original water is left

I am assuming we all agree that 50% changes more water than 20%?
Please note: The lower the resulting number in this calculation (which is the amount of the original water left) the more water is changed.

Now back to the question:
4 x 20% changes
(1-.2)*(1 -.2)*(1 -.2)*(1-.2) = .4096
(Only 41% of the original water is left)
1 x 50% changes
(1-.5)^1 = .5
50% of the original water is left.

So 4 x 20% changes more water than 1 x 50%

In general to find out how much of the original water is left, use the following formula:

(1 – A)*(1 - B)* (1 – C) * …
Where A, B, C, … is the % water changed divided by 100.

If you always change the same amount of water then the expression simplifies to:
(1 – A)^N
Where A is the % water changed divided by 100.
And N is the number of changes.

Which plan is more work is a relative thing.
If you have a 10 gal tank changing 50% means only changing 5 gal of water. Making, conditioning, adjusting and handling 5 gal is manageable for most people.
If you have a 100 gal tank, changing 50% means changing 50 gal of water. Making, conditioning, adjusting and handling 50 gal is going to require a lot more concentrated effort and even specialized equipment. You probably would want to do more frequent smaller changes.

Changing water is the best thing you can do for your fish/plants. It is also the riskiest and the more water you change at one time, the greater is the chance of making a fatal mistake.
For example:
Say your make up water is 22°C and your tank water is 32°C. If you do a 50% water change the new tank temp is:
.5 * 22°C + .5 * 32°C = 27°C (A change of 5°C)
A 10% change results in:
.1 * 22°C + .9 * 32°C = 31°C (A change of only 1°C)
So be very careful changing water!
I use this, took me 1.2 hours to change 60% on 440 Gal worth of tank over 5 tanks, I cleaned the filters, trimmed, etc while the water drained, and filled.

Hooks up to the shower.

Hangs on the tank, the other end drains and irrigated the yard.
That same end goes on the shower head to refill, add some dechlor, I'd done. Nice 3/4 Flexogen hose, never kinks........3-4\$ worth of PCV...........

Water changes are only as hard as you want to make them.
Some are downright easy if you hard plumb them to a drain and refill valve and add activated carbon for the refill.

Simple easy solution and quite cheap.

#### Sharkfood

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As far as one over the other's effect on dilution, 4x20% water changes remove 10% more solids for only 4X the work. I don't know many people other than discus breeders who change their water nearly every day, but if that sort of thing is what floats your boat, then sail on.

Of course the point becomes completely moot if your once a week, large water change is more than 59%.

So I submit that changing 61% of your water weekly is superior.

Now what you should do is make a post arguing that 7x20% water changes in a week is a better method, to which I will counter that changing 75% of your water at a time, once per week is better. Maybe we could even go so far as to argue that changing 20% twice a day is the real way to go.

Can you see where I am going with this?

#### shane3fan

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I think the 50% /week water change is a good rule of thumb for a tank that requires frequent changes ( not a natural Walstad style tank ). However, I think the closest thing to 'perfect' is an automatic, constant water change system with a drip/drain setup. This is what I have planned for my 180 gallon tank.

#### plantbrain

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As far as one over the other's effect on dilution, 4x20% water changes remove 10% more solids for only 4X the work. I don't know many people other than discus breeders who change their water nearly every day, but if that sort of thing is what floats your boat, then sail on.

Of course the point becomes completely moot if your once a week, large water change is more than 59%.

So I submit that changing 61% of your water weekly is superior.

Now what you should do is make a post arguing that 7x20% water changes in a week is a better method, to which I will counter that changing 75% of your water at a time, once per week is better. Maybe we could even go so far as to argue that changing 20% twice a day is the real way to go.

Can you see where I am going with this?
Nit picker:tongue:

How dare you suggest my % WC's are not precise measures of the data, you found me out.

It sort of all comes back to what is really the risk of the build up and toxicity to a variety of plants, algae, livestock, shrimp etc. No one has done that, so the debate is a bit silly without some end point goal.

Automated systems I've installed for 3 client's, and the float switch does the rest, one TPT member dialed his phone to change his water at home while at the AGA convention..........

All the techys were all "OOOO....... ahhh"

Water changes are a useful tool for managing nutrients, I'm not sure there's any real evidence that more or less really helps, but it does make the ranges more known and stable.

I'm not sure stable is really good or bad, I've not seem much evidence there either.

But..........it does not take long to do 61% or 80%, vs 50%.

I just chose 50% to simply the max build up from dosing for the week, and many discus folks typically already do 50% 1-2x a week. Nothing can build up more than 2x the dosing for the week. That's simple and easy to understand.
At the time, I was breeding them and noticed the tank did really really well.

If you run the dosing EI modeling calculator, you will not that more frequent smaller changes keep the tank more stable ppm's wise..........but the question is ..is that better and if so, how? What evidence is required to demonstrate that? I've looked for oh, 15 or so years and have not come up with any decent reason I could verify.

And could you do 25% instead of 10% or 20%?

Sure, but if you wanna drag out the hose and do the water change.........it simply does not take that much more labor or time to change 20% vs 50%.
Like I said a long time ago, if you bother doing them, may as well go big.
No worming around the common sense and the human motivational part there.

for those wanting to invoke the "Well it works for me" or "I'm just sharing and different method"...here's my response to that and it requires Zero water changes, no \$\$\$ cO2, and no labor to speak of, feed the fish(So I do technically dose) and that's pretty much it, bred dozens of fish in here too:

Show me your tank that has no water changes for 6 months?
12 months?

I mean if "less is better" and all..........

Go whole hog, stick with a philosophy, and run with it........ this ain't a buffet, you do not get to pick and chose to suit.

#### shane3fan

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The thing that takes the longest for me is refilling the tanks. I did maintenance on 10 tanks Monday night at my house ( water changes, cleaning, pruning etc ) and it took a little over an hour and half---most of that was waiting for the dang water to refill the tanks.

#### plantbrain

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The thing that takes the longest for me is refilling the tanks. I did maintenance on 10 tanks Monday night at my house ( water changes, cleaning, pruning etc ) and it took a little over an hour and half---most of that was waiting for the dang water to refill the tanks.
Clean the filters, wipe the glass, feed the fish, add ferts, stretch your legs and back, lift some weights, eat dinner.........

#### ray-the-pilot

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Clean the filters, wipe the glass, feed the fish, add ferts, stretch your legs and back, lift some weights, eat dinner.........
I could be wrong (and as I said I often am) but are you saying that you just dump whatever comes out of your city water pipes into your tanks?
What if anything do you do with city water?

#### VeeSe

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I could be wrong (and as I said I often am) but are you saying that you just dump whatever comes out of your city water pipes into your tanks?
What if anything do you do with city water?
Most people just take their tap water straight into their tanks (plus adding some dechlorinator to it). I'm not sure if I'm interpreting what you're asking correctly, but if you're asking if people just put what comes out of their sinks/hoses directly into their tank, then yeah, I think you'll find that most of us go straight from tapwater outlet to tank with a hose while adding dechlorinator to the tank.

#### Hoppy

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Plain tap water is the norm except for the few who have atrocious tap water, or a very special need. Why complicate something unnecessarily?

#### Chlorophile

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I've been doing 5 gallon buckets for the entirety of time I've enjoyed aquariums.
I have never once had a tank over 29 gallons simply because it hasn't appealed to me often, although I would love a 90P.
However, since I have always done water changes with 5 gallon buckets I've always added dechlorinator to each bucket; I wonder, is their really no benefit to dechlorinating water before you add it to the tank, or should I be adding the dechlorinator to the tank before I add the water?

I am aware that the removal of chlorine happens quite quickly with dechlorinator as it bonds to chlorine to make sodium chloride (salt), however I am unaware as to how it removes chloramine and heavy metals.
In the situation where you do not dechlorinate your water ahead of time is it a good idea to not run the filter for a while to prevent chlorinated water coming into contact with the bacterial populations in the filter?
As I said I have never, ever, added water to the tank that wasn't dechlorinated first - however my low-tech tank hasn't had a water change in 9 months, but I still top up with dechlorinated tap water.

Also, I'm with plantbrain on the water change frequency thing. In my high tech tanks I do a 50ish percent water change weekly but that's only to reset any excess in nutrients.
Like I said my lowtech tank hasn't had a water change in 9 months and the only negative factor I can see is that I have fairly bad GDA, but I don't really care, my plants are growing perfectly slow so the most maintenance I have is feed the fish and stare at the tank. I don't even clean the GDA anymore.
I really like the tank you pictured, plantbrain, the above water plants is a great idea!
I can't see any livestock in those photo's so I assume you're dosing, at least very infrequently, but that's a brilliant idea for a tank that you don't want to dose or inject with co2. You still have plants, you have extra "biological" filtration as far as the crud in our water, and the maintenance should be nearly none. But you already knew that - no water changes!

#### VeeSe

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I've been doing 5 gallon buckets for the entirety of time I've enjoyed aquariums.
I have never once had a tank over 29 gallons simply because it hasn't appealed to me often, although I would love a 90P.
However, since I have always done water changes with 5 gallon buckets I've always added dechlorinator to each bucket; I wonder, is their really no benefit to dechlorinating water before you add it to the tank, or should I be adding the dechlorinator to the tank before I add the water?
I just add enough dechlorinator to treat the whole tank volume (I add 5mL of Prime for my 55G when 1mL treats 10G of water) before I start dumping in water. I used to do the same as you when I was lugging buckets. I did about 5 or 6 weeks of lugging buckets for my 55G before I was like "this sucks so much" and got an automatic water changer. Lugging 40 lbs of water back and forth sucks a LOT compared to just setting up your hose and seeing it go.

#### Chlorophile

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I just add enough dechlorinator to treat the whole tank volume (I add 5mL of Prime for my 55G when 1mL treats 10G of water) before I start dumping in water. I used to do the same as you when I was lugging buckets. I did about 5 or 6 weeks of lugging buckets for my 55G before I was like "this sucks so much" and got an automatic water changer. Lugging 40 lbs of water back and forth sucks a LOT compared to just setting up your hose and seeing it go.
.....wait a minute - you add enough dechlorinator to dechlorinate the entire tank? I think I heard one other person say that they did this as well.
Is this common for most of you? To treat the entire tank as opposed to the amount of water you are adding?
On my 20 gallon I add .5 ml per 5 gallon bucket(1ml treats 10 gallons).
If I were to dechlorinate at your dosage but still by bucket method I would add 1ml to both buckets. Or simply 2 ml to the tank and pour in the chlorinated water.
Why do you add so much dechlorinator?
I'm not in the mindset of telling myself I should avoid manual labor, I fear that's how most of us in the states get fat and lazy. The harder the better. After all a rolling stone gathers no moss.

#### NeonFlux

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I have several questions about some things involving WC's...

It may be a bit off-topic, but what if you don't change your tank's water for months in a high tech heavily planted MTS tank (inspired by AaronT) with t5's, co2 and a bit of NPK and trace from dosing and fish's waste... Now if you decide to just top off the water that was loss by evaporation, and keep the tank going without WC's, wouldn't there be a big build up of fish hormones excreted by all of the fish in the aquarium eventually? Would the plants do the natural job of absorbing it? What about silicates and organics? Would they too be absorbed by plants? Or can they only be removed by WC's only?

For one of my MTS tanks, all I do is top them off whenever a large amount of water has evaporated.. but I do WC's once every few months, the plants take care of the waste... There is a good amount of fish in the tank too.

#### nonconductive

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i havent made it to 6 months. but i have gone up to 3 with no problems. once i get rid of the large poop makers i probably could go atleast 6 to a year. need a pic? check my sig.

EDIT: sometimes i forget i have more than 1 tank. so yes i have gone atleast 6 months with just topping off.

#### ray-the-pilot

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Most people just take their tap water straight into their tanks (plus adding some dechlorinator to it). I'm not sure if I'm interpreting what you're asking correctly, but if you're asking if people just put what comes out of their sinks/hoses directly into their tank, then yeah, I think you'll find that most of us go straight from tapwater outlet to tank with a hose while adding dechlorinator to the tank.
So if your tank is at 80deg F and your water (like mine is in the winter) is at 50 deg F. You don't worry about the temperature change on your discus?

#### ray-the-pilot

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Plain tap water is the norm except for the few who have atrocious tap water, or a very special need. Why complicate something unnecessarily?
OK you are saying that I should take my 50 deg F tap water and just dump it into my 80 deg discus tank?

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