Water change math

[jasonjnj]

Howdy

This is just to check my own sanity. I see lots of threads where people, during their initial cycle, will say something like "This morning, my ammonia was at 5. So I did a 20% water change and brought it back down to 1."

Say what? If the ammonia is evenly spread throughout your tank at 5ppm and you remove 20% of the water, you're removing 20% of the ammonia. So after adding back the water you removed, you should have 4ppm in the tank.

Am I missing something here or are people just completely misreading their test kits?

--
Jason

 

[reef-man_d-man]

i dont think it works that way, first you removed at least 1 ppm then you diluted the rest out thus lowering the count even lower than it was, its kinda weird to think about but I think thats the way it works.

 

[nickb]

THe 4 ppm is right.

Say you have a tank with volume 'V'. Then, ignoring the units , the amount of ammonia in the tank is 5V. If you remove 20% of the tank water, the amount of ammonia left is 4V (0.8 X 5V). You add back 20% of water volume with fresh water and the concentration is now 4V/(V) = 4 ppm.

You do remove 1ppm but only because you removed the water it was in. That will not change the concentration at all, until you add new water to dilute it down..

I am sure that doing the water change will impact on the biology of the tank but that is a longer term effect. The dilution effect can not bring 5 ppm to 1 ppm (unless you do a 80% water change).

 

[kp1917]

Can we get some more input here? The logic sounds good but it sure seems to me that a 10 to 20 percent water change knocked down my ammonia by a lot more than 10 to 20 percent when I was having large 4+ amm. readings.

KP

 

[jasonjnj]

Not much to discuss, methinks

Not much to discuss, methinks

It boils down to simple math. You have 5 parts ammonia for every million parts water. Say your
10-gallon tank amounts to 50 million units of water. You have 250 units of ammonia. You remove 2 gallons of water, which removes 50 units of ammonia along with it.

Now you have 200 units of ammonia. You add back 2 pure gallons of water, so you're back up to 50 million unts of water and still have 200 units of ammonia. 200/50 = 4 parts per million.

 

[PRC]

jason is correct
This should probably move to the reef chemistry forum where Randy may be able to shed more light on it. It won't get much attention here.

 

[hdtran]

KP,

You may have had a coincidental reduction of ammonia due to the bacteria taking off at the same time as the water change.

Jason & PRC are correct. Given no bacteria, a 20% water change will only reduce your ammonia from 5ppm to 4ppm.

It is equally possible that your test kit is nonlinear (e.g. above 1ppm, readings are less accurate).