How sure are you of your total system volume?

[Original Fin]

I thought I was in the ballpark with my estimate. Wrong!
Just thought I'd share this...

I've recently switched from Kent AB Ca and Alk buffers to Bulk Reef Supply 2 part recipe 1. Using their online reef calculator to dose did not work out as expected...it overshot the mark by quite a bit.
Here's why, I think. The system volume I plugged into the calculator was 65 gallons. I have a 55 gallon tank, and two 10 gallon sumps. One of those sumps is only half full, so my rough estimate has always been about 65 total system gallons.

My starting level for Ca was 380, and I wanted to get to 429. The calculator came up with 326 ml of solution. When I tested 24 hours later, Ca was at 460...an overshoot of 31ppm.

My starting level for Mag was 1290, and my target was 1370. The calculator came up with 419 ml of solution. After 24 hours, it tested at 1440...an overshoot of 70ppm.

Afterwards, I figured that my estimate of 65 gallons may have been a bit high. I should have subtracted for rock, sand and equipment displacement. How much could this be though? A few gallons? Nope. Much more.

So here's how I figured it out. Since I knew how much the original recommended dosage actually raised levels by, I would use those figures (CA base 380 to actual 460, and Mag 1290 base to actual 1440) and plug them into the calculator, then enter progressively lower system volume (from 65) , until the new recommended dosages matched the original ones.

The results

For Calcium, a desired 80 point increase with 326ml of solution would require just 39.8 gallons!

For magnesium, a desired 150 point increase with 419ml of solution would require just 34.7 gallons!

I've not tested this equation with the BRS Alk solution yet as I'm still finishing up my last bottle of Kents B, but I assume it will be similar to what I found with Ca, considering they are supposed to be balanced solutions.

I have revised my estimated total system volume to 40 actual gallons of water, so from all of this I can deduce one of two things: Either the BRS salts and calculator are not properly calibrated from their end, or my 70 lbs of rock, 80 lbs of sand, and various equipment must be displacing about 25 gallons of water, or 38% of the total capacity!

[chrisfont23]

Sounds like the displacement is a factor. It shouldn't make a difference but who knows what went into the calculation in the first place. Stupid physics!

[TimH07]

volume calculator

Aquarium System Volume Calculator

[Original Fin]

Thanks Tim. Ive used his chemistry calculator before, but never checked out his homepage where the link to this is.

Ive also used other dimensional tank volume calculators, but this is the first I've seen that attempts to compensate for additional plumbing and displacement of sand and rocks.
I already knew that between my DT and two 10 gallon sump tanks, the max water volume with nothing else in the tanks is about 70 gals...baffles in the sump and the level I run in the return chamber loses about 5 gallons. I ran the numbers on it, and it only deducts about 8 gallons for 70lb rock, and 80 lb sand. (60 DT & 20 Fuge)

There's also the displacement of two Koralia power heads, an internal overflow weir, heater, and return pump. No calculator could give an accurate displacement estimate on those items, but I doubt that it comes to the missing 17 gallons from my estimate based on dosages. In my mind, this points the finger squarely at the BRS mix recipes being too too potent for some reason.
Since these batches are already mixed, I'll just continue to use the modified system volume values I came up with.

[Jessie Nicoletti]

Hey original hope this dosent confuse you as some of my posts have i hope this actualy helps you
i havent done this since high school chemistry lol so my calculations can posibly be off

get a 5 gallon bucket CLEAR ( tall not flat) fill it halfway with water mark off the water level
take a 2-5 lb piece of live rock put it in the bucket (A) . mark off the water level again
siphon off the water between the 2 lines and put it in a measuring cup (X)

X = the amount of water displaed in the bucket A = the wieght of the rock used
Z = the amount of water displaced in the tank B= total wieght of all rocks in the system ( i wouldnt include sand it displaces water diferently)

you want to find out Z once you have A divide by B and multiply by X this should give you Z
you may have to conver gallons to ml or whatever the mesuring cup uses or vice versa


dont forget to consider denser base rock will displace alot more water then say branch rock or decorator rock ect id be willing to bet that with the rocks and sand and equipment you displace maybe 20 gallons so it may be less water volume then the tank even also the potentency or concentraiton of what you add can be affected by how long its been on the shelf and if the container sits open , and even tempeture can affect the concentration value.
now a heater and korilla power heads shouldnt even displace 1 gallon of water by themselves. btw

[Matities23]

I love messin around with those things... only problem they are almost never right... IMO all tanks are different.. some might take more Ca than Alk.. or the other way around... Plus also remember that the chemical concantration of both brand might be different.. not sure since the last time I use 2 parts it was B-ionic. For next time if you are changin from brands.. dose 1/3 less of usual amount and check levels 2 or 3 time weekly till you know how much ur tank is consuming.