Water change volume

D. Torres · Mar 14, 2018

I have a standard 75 gallon tank. I was wondering if I need to take into consideration the volume displacement of my live rock and sand when calculating the percent of water to change. For example 20% of 75 is 15 but with 85lbs of live rock my actuall water volume is only 62 gallons and 20% of that is 12.4. I know it' only about a 2.5 gallon difference but I just don' want to mess things up.

Additional info: I only plan on doing a water change every 2 weeks. Using RO/DI water and reef crystals

Deezill · Mar 14, 2018

This is a great question and I am going to follow this thread because I wondered the same thing. I have a 150g with 165 I need to figure
my actual water volume and not to mention I have 130 lbs live sand. So I am guessing we should consider the displacement fact. maybe there is a calculator somewhere so we can get true numbers.

D. Torres · Mar 14, 2018

I used this calculator, but use with caution because I'm not sure of it's accuracy.

http://www.garf.org/calculators/TankVolumeCalculator.asp

Deezill · Mar 14, 2018

Thanks much checking this out now.

nautical_nathaniel · Mar 14, 2018

I do 25% water changes by total tank volume on my tank weekly, even though my tank probably has 16-18 gallons of water actually in it after you account for rocks, corals, and equipment. It doesn't affect things all that much in my experience. Also, 15 gallons is a lot easier to measure out and do salt calcs on than 12.4 is. I don't think you're going to mess things up if you end up doing more or less than 20% with every water change as long as you're consistent to some extent (i.e. using the same container or having the same salinity with every batch) and do water changes at regular intervals.

Randy Holmes-Farley · Mar 14, 2018

D. Torres said:
I have a standard 75 gallon tank. I was wondering if I need to take into consideration the volume displacement of my live rock and sand when calculating the percent of water to change. For example 20% of 75 is 15 but with 85lbs of live rock my actuall water volume is only 62 gallons and 20% of that is 12.4. I know it' only about a 2.5 gallon difference but I just don' want to mess things up.

Additional info: I only plan on doing a water change every 2 weeks. Using RO/DI water and reef crystals

It doesn't really matter because there no magic number of what size change is optimnal. Folks change from none to a lot (at least on small tanks).

When I estimated my change (~1% daily), I was referring to estimated tank water volume, but the actual amount my pump was changing for me could easily have been twice that or half that at any given time. I just went by the rated delivery rate vs minutes on a timer, and never actually measured it.

potatocouch · Mar 14, 2018

Doesn't matter what percentage .. for me, what matters is salinity, temperature and the amount you replacing should equal to the amount you dumping.

Deezill · Mar 14, 2018

potatocouch said:
Doesn't matter what percentage .. for me, what matters is salinity, temperature and the amount you replacing should equal to the amount you dumping.

Agreed.

savetheocean · Mar 14, 2018

I do 10-15 gallons every weekend.

neupane00 · Mar 14, 2018

Doesn’t really matter!! I have done 5 to 20% weeky. It’s more art than science

NY_Caveman · Mar 14, 2018

Agree with the above. Just good to know what actual water volume you have (as you do) if you start trying to calculate specific parameter differences of say calcium or alkalinity.

EDIT: differences between the tank and the new water.

Susan Edwards · Mar 15, 2018

Here is another calculator. http://reef.diesyst.com/volcalc/volcalc.html

I just estimate and go by how much water I pull out in my 5 gal or 10 gal buckets. Don't think you need to worry about being exact.

Bpb · Mar 15, 2018

A better way to get your actual tank volume is to measure your calcium level.

Dose a small amount of calcium (i use BRS CaCl solution since they have a simple to use calculator), something that would give an estimated 10 ppm increased based on your estimated water volume.

Dose that, wait an hour for it to circulate. Test calcium again

Now take your initial calcium level, and plug it into the BRS calculator, and take your second reading and plug it into the “desired level” section.

Now just change the tank volume and keep calculating until the instructed dose matches what you actually dosed. There’s your actual real net water volume.

For my 90 gallon display, with 36 gallon sump (which is only filled 1/2 way, we obviously don’t top off our sumps), I am at 86 net gallons of water.