% Water Change Formula

Ricochet

Exodon
MFK Member
Dec 28, 2009
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Siam
Years ago, I remember there being a formula for tank water changes...

For example if you change 20% of your 100 gallon tank every two-three days after 5 water changes over 10 to 15 days you will not have entirely changed and/or rotated all of the older water out of your tank. It would only be 68% or so...

There was a formula that would allow one to determine the number of water changes at a specific percentage required to nearly (closely) cycle the entire tank. Of course within reason as one may be stuck at 99.9% for a long while.
 

mgk

Feeder Fish
MFK Member
Jan 2, 2010
626
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nowhereville
20% change every day
start; 100% original water
day 1:80% left original water
day 2:64% left original water
day 3:51.2% left original water

im taking 20% of the % left each time....


the whole thing is absolutely useless as the fish produce waste all the time.

get yourself a nitrate test kit and decide how much water to change by that.
 

kdrun76

Piranha
MFK Member
Mar 4, 2009
1,637
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CT
To cycle a tank?


Do you mean to change all the water? If you want to have changed all the water, the first thing you need is a time frame.

Lets say you wanted to have as close to 100% of the water changed out in a month.
Week 1-50% change. (Changes 50% of the water for the month)
Week 2-50% change. (changes 75% of the water for the month. The old 50 + 50% of the new change.)
Week 3- 50% change. (changes 87.5% of the water for the month. The old 75 + 25% of the new change.)
Week 4- 50% change. (Changes 93.75% of the water for the month. The old 87.5 plus 12.5% of the new change.)
A 5th 50% change will give you a total of 96.875% changed.

IE: 5 water changes of 50% is a 97% water change in that time frame. A 6th change is 98.4% (Not enough different to care about!)


Ok, where is the formula? You MUST have a time frame. No other option.
In side that time frame... you change X% and x% is the value of the first change.

Change 2.... you now have 100-x% of "old water" left in your tank. So your water change will only change a percentage of the old water that is left. If you followed the 50% water changes I suggested above. After the first change you only have 50% of your tank with old water in it, and you changed half of that.... meaning you only changed out 25% of the old water... a total change of 75%.

The third water change will change 50% of the old water that remains, in this case only 25% old water remains so you are talking about a 12.5% water change. and so on.

The real problem is that the fish are constantly replacing the waste you are removing, stick with 50% a week as long as that keeps your nitrates below 20ppm. More often if it doesn't.

I hope that helped.
 

Nikko112

Feeder Fish
MFK Member
Jan 1, 2010
72
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Long Island
much agreed with MGK. i do weekly 25% water changes just to make sure ammonia and nitrate levels are at 0%, but still testing water.
 

kdrun76

Piranha
MFK Member
Mar 4, 2009
1,637
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CT
Nikko112;3780337; said:
much agreed with MGK. i do weekly 25% water changes just to make sure ammonia and nitrate levels are at 0%, but still testing water.
If your ammonia and nitrite levels aren't zero, you aren't cycled and you should be changing a LOT more water... like 50% a day if needed.

After the tank is cycled...you still need to change 50% a week.
 

Ricochet

Exodon
MFK Member
Dec 28, 2009
33
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21
Siam
Ohhh, I will continue to test still. Should have stated that before.

I just couldn't remember the formula that was kicking around for water chages and I understand completely about the continual contamination from waste. Working on an idea for a DIY project and the ole gray matter failed me.

Thanks mgk and kdrun76 that was what I was looking for.
 

vfc

Candiru
MFK Member
Jan 25, 2007
695
3
48
Philadelphia
I made this Excel spreadsheet a couple years ago to help me select the optimum water change schedule.

The attached example is a 75G tank with a weekly 50% water change with 5PPM nitrates out of the tap.

75G Water Change  7-Days 50%.JPG
 

dawnmarie

Feeder Fish
MFK Member
Sep 21, 2009
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California Delta
www.eddosharbor.com
vfc;3781122;3781122 said:
I made this Excel spreadsheet a couple years ago to help me select the optimum water change schedule.

The attached example is a 75G tank with a weekly 50% water change with 5PPM nitrates out of the tap.
Nice spreadsheet. If it is scaleable to tank size and fishload this would be a great sticky. How can I aquire a copy ?
 

aquaculture

Feeder Fish
MFK Member
May 5, 2009
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In search of Leiarius
I think the formula would be good for those that have nitrates being eliminated by bacteria or plant life, and would just need to change water to keep it fresh and not stail.
 
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