Water change volume question

[MikeyZo]

So I have a random question. When you do a water change, say 20%, are you calculating on the total volume of the tank or the actual water volume?

So for instance, 100 gallon system (call this the theoretical volume), 20% is 20 gallons.

But you have rock and coral etc, so let’s say the true volume is 80 gallons, that’s 16 gallons. I know it’s not a huge difference but just curious what everyone does.

Same goes for dosing calculations. Are you doing total theoretical volume or the true water volume?

 

[commod0re]

it’s too difficult to calculate out the volume of all the stuff in it for this purpose. I have a 26 gallon tank so when I do my 6 gallon weekly water changes I say that’s a 23% water change (6/26=0.23)

I haven’t gotten into any dosing yet so I have no experience to back this up but my feeling is that it’s similar there - estimating the total volume of all of the non-water stuff in the tank is difficult so instead you use the total tank volume combined with parameter monitoring in order to dial in the dosages

 

[LavenderTi]

I use actual water volume. It’s a value I have an approximate idea of since I need it for calculating dosages and such.

 

[Crabs McJones]

I always went off total system volume. So I had a 40 breeder with a 20 sump, so I would do around 6 to 7 gallons a week.
When I had my 75 with a 20 gallon sump I rounded up to 10 gallons a week