Do you go off of raw tank volume or actual tank volume when dosing supplements?

[jcnkt_ellis]

Ok, I was feeling nerdy and while calculating the needed salinity level of my water change to raise the tank from 1.023 to 1.025 I worked out a formula, attached as a pic, to determine that in actuality my 150g DT + 30g Sump only had 98g of water in it. In other words, my rock, sand, skimmer, etc...was displacing 82g of water. So that had me thinking, when I dose supplements do I use the 180g total, like I have been, or do I reduce the amount to whats appropriate for 98g?

Tank Volume Formula.jpg

[rrasco]

Typically you want to account for displaced water. I have a 105 with a 40 breeder sump and do my calculations based on a 100g tank. If I were to put 145g worth of salt mix in my tank it would be dosing too much. I'd say your thinking is spot on.

How did you determine your displacement? I just ball-parked mine.

[Big_Pun]

guess i would have to add reactors

[jcnkt_ellis]

Okay, the picture linked is the formula I used to determine how many gallons of water my tank held. It requires a change in the salinity to work. You know the current salinity of your tank, you know the amount of water you removed, you know the salinity of the water you are putting back in, and you know the final salinity when everything is said and done. Using those knowns you solve for the unknown, the total water volume in your tank. This formula would account for any additional equipment you have hooked up to the system. The output is the amount of water actually in the system after all displacement is accounted for.

[jcnkt_ellis]

To determine actual displacement rather than current water volume would be much more difficult as most of us ballpark the amount of water in our sumps, reactors, overflows, etc...when determining the volume of the system without any displacement (i.e. a bare tank).

[jcnkt_ellis]

I feel like I may be rambling, but the formula was derived from the formula used to calculate average salinity of your tank:

{[current salinity]*(volume of tank - volume of water change) + [salinity of new water]*(volume of water change)} / (volume of tank) = (salinity of tank after water change)