A Note On Water Changes - Why Big Numbers Are Important

[dannybgoode]

There's been a few posts recently about how much water to change, are large changes safe, what to do when ammonia or nitrite levels are high (and I'm sure there always will be these type of questions - its fundamental to our hobby) so I thought I'd bring a bit maths to the table to show why large water changes are important to keep all the parameters in check.

Imagine you have a situation where you find your nitrAtes are a bit high - 50ppm. So over the course of the next 5 days you change 20% of the water every day.

Assuming your tap water has 0ppm nitrAte (an unlikely scenario but more on that in a bit) at the end of the 5 days, and not taking into account the nitrAtes that will go into the water as part of the bacterial cycle you will still have a reading of 20ppm.

With this is mind, it is quite conceivable that if (as many people do) you are only doing a 20-30% water change every week, over time your nitrAte level may not drop at all and could even increase, particularly given that most tap water does contain some nitrAte.

Now, nitrAte isn't so bad for your fish and regular water changes generally keep things in check but what it you have an ammonia spike? It has been the topic of many a post - q. 'I have an ammonia reading of 4ppm, what should I do?' a. 'Big water changes - 80%+ daily until things stabilise...'

So lets look why this advice is given and why such large changes are recommended. You have an ammonia spike of 4ppm, you ask the question and end up giving it a go but only manage change 50% of the water every day for 5 days.

Where does this leave you?:


Start 4ppm WC 1 2.666667ppm WC 2 1.777778ppm WC 3 1.185185ppm WC 4 0.790123ppm WC 5 0.526749ppm

So, even without the ammonia your fish are adding to the tank and even after doing 50% water changes for 5 consecutive days you are still over the generally accepted 'safe' level of .5ppm (I use the term safe loosely - 0 is always best!).

Lets look at the same regime but with 80% changes:

Start 4ppm WC 1 2.222222ppm WC 2 1.234568ppm WC 3 0.685871ppm WC 4 0.381039ppm WC 5 0.211689ppm

Not until day 4 do levels drop below ,5ppm and that's at 80% changes and without factoring the ammonia being added all the time to the water.

Taking the ammonia being added constantly, even after day 5, to keep on top of things you would have to keep these big big changes up.

I hope this explains why big changes are good (if not sometimes essential) and so long as you match the temp and decholrinate it shouldn't be stressful for your fish - in fact quite the opposite.

Cheers

Danny B

 

[Mikey1]

thats alot of good info, but all the average fish keeper (novice to intermediate) would hear out of that is "blah blah blah"

i think its more wise to just recommend the average fish keeper to change 50% once a week, thats a reasonable amount for the average stocked and fed aquarium

 

[dannybgoode]

Depends how willing they are to learn! Under normal circumstances 50% weekly is probably fine (unless you're into Discus).

Its more when there's an ammonia or nitrIte issue that this information becomes more pertinent.

Cheers

Danny B

 

[zain611]

this post should be pinned

 

[MrsM]

All i heard was blah blah blah.....

 

[DreamertK]

I've been doing 50% every day for the past few days to lower my nitrites. Your math explanation kind of confused me because of poor layout (everything is smashed together, making it hard to read).

Like this for example:
Day 1: NitrAtes 40ppm
WC 50%
Day 2: 20ppm
WC 50%
Day 3: 10ppm...ext

Day 1: NitrAtes 40ppm
WC 80%
Day 2: 8ppm
WC 80%
Day 3: 1.6ppm...ext

When you're working with small numbers for ammonia/nitrites, yes it doesn't affect it as much to do 80% rather than 50%.

 

[MrsM]

I've been doing 50% every day for the past few days to lower my nitrites. Your math explanation kind of confused me because of poor layout (everything is smashed together, making it hard to read).

Like this for example:
Day 1: NitrAtes 40ppm
WC 50%
Day 2: 20ppm
WC 50%
Day 3: 10ppm...ext

Day 1: NitrAtes 40ppm
WC 80%
Day 2: 8ppm
WC 80%
Day 3: 1.6ppm...ext

When you're working with small numbers for ammonia/nitrites, yes it doesn't affect it as much to do 80% rather than 50%.

Ahhhhh ok i get it now, lol.....my tap water has 20-25ppm nitrate.....so my tank is generally quite high.....am i right in thinking live plants would lower nitrates? But when it comes to WC day...would the high nitrates from the tap not shock the fish?
So...if i had a lot of live plants in my tank(lowering nitrates) im assuming smaller WCs say...twice a week is better than a 30% WC once a week?

 

[dannybgoode]

Yeah, sorry about the formatting - it looked ok when I previewed it and then messed up when it posted.

You see, you are making the mistake of thinking 40, 20, 10 on the nitrates - it goes:

40
26.6
17.7
11.85
7.9
Etc

Its an exponential thing...

Cheers

Danny B

Cheersd

 

[DreamertK]

Exponent? Not a percentage thing? Since you're taking out 50% of the water which contains 50% of the nitrates, wouldn't you be removing that amount? Or is the exponent factoring in the amount being produced by the fish?

 

[ColR1948]

I made a post a few days ago about large water changes effecting the fish, I would like to point out if you read it properly I DID say unless there was a spike in which case a large WC would be good.
The post i made was about someone who had never done a water change so the change was a shock, (just setting the record straight.)

 

[dannybgoode]

Oh, this wasn't a dig at you at all-it was a general point. Yes, there are certain circumstances when large wc's cause an issue such as the one you described.

It was to point out why big wc's are important in certain circumstances...

Cheers

Danny B

 

[tata]

Yeah, sorry about the formatting - it looked ok when I previewed it and then messed up when it posted.

You see, you are making the mistake of thinking 40, 20, 10 on the nitrates - it goes:

40
26.6
17.7
11.85
7.9
Etc

Its an exponential thing...

Cheers

Danny B

Cheersd

Can you please explain the math behind these figures?

I expected that 50% of water with 4ppms + 50% of water with 0ppms result on 2ppms water, but according to your explanation is not the case.

 

[ColR1948]

Not getting at the poster of the thread but talking in general this water change business certainly is a subject that causes debate and many posts.

For the newby to the hobby getting across that thay must do water changes can be very hard in SOME cases, I admit some have never been told and find out when it's too late.
For those that do find out and ask about the changes this too can be a bit daunting, again they will get conflicting advice, I know we all mean well in advising but I and I'm sure you all have read, in other posts where someone will say, do 50% then another will say 75% then another only 25% and the frequency of the changes too can vary, from once a day to twice a day to once a week and so on.

I know too there are many different cases, as mentioned above about if there was/is a spike and a change is needed quick and frequently to the normal cases of the weekly change.

Then there is the problem of keeping certain fish in certain types of water like soft or hard, adding buffers to alter it, it goes on and on but this is the hobby we have chosen, I'm still learning and slowly but surely getting better.

It would be nice to hear some of the the fish shops actually asking people if they are new to the hobby and giving CORRECT advice instead of just selling them any fish and telling them the WRONG information, I once did this in a fish shop and was told my the owner to mind my own business, but he was giving wrong advice just to make a sale, sorry I'm changing the subject, didn't mean to.

 

[dannybgoode]

I'm not ignoring people, I'm just at work at the moment and will explain when I have a minute...

Cheers

Danny B

 

[DreamertK]

I've been looking forward to your explanation.

I was under the impresssion that it was a matter of ratios this whole time. Usually exponential factors are reserved for bacterial growth/halflives of decaying atoms.