How do you estimate the actual water volume in your tank?

[Vincerama2]

I decided I better keep up on alk & calcium, etc, so I went to use the BRS online calculator for how much CA/ALK solution to add to the tank and of course, I know how large my tank and sump are, but not everything is water.
I estimated, then used the calculator and dosed alk (soda ash) and the alk went beyond what I expected. I'm not expecting the calculator to be dead on accurate because a lot of stuff is going on in the tank, but I wasn't expecting what I got when I tested for alk. I dose what the calculator recommended for a 160 g of water (I have a 180, with a not-quite-full 29 g sump). And the alk went quite a bit higher. I fiddled with the numbers, using what I dosed and what alk I ended up with and reverse engineered that what I dosed would have brought me up to what I measured if my tank had 80 gallons of water in it.
Now, that's not exactly an accurate or precise way to measure water volume, sure... but for the purposes of dosing, I'll use 80 the next time I adjust my alk or CA to see how accurate a guestimate it is. You only need to know water volume for dosing and maybe water changes.

Is there a better way to calculate water volume aside from just draining every drop out and measuring?

V

 

[RandyC]

I decided I better keep up on alk & calcium, etc, so I went to use the BRS online calculator for how much CA/ALK solution to add to the tank and of course, I know how large my tank and sump are, but not everything is water.
I estimated, then used the calculator and dosed alk (soda ash) and the alk went beyond what I expected. I'm not expecting the calculator to be dead on accurate because a lot of stuff is going on in the tank, but I wasn't expecting what I got when I tested for alk. I dose what the calculator recommended for a 160 g of water (I have a 180, with a not-quite-full 29 g sump). And the alk went quite a bit higher. I fiddled with the numbers, using what I dosed and what alk I ended up with and reverse engineered that what I dosed would have brought me up to what I measured if my tank had 80 gallons of water in it.
Now, that's not exactly an accurate or precise way to measure water volume, sure... but for the purposes of dosing, I'll use 80 the next time I adjust my alk or CA to see how accurate a guestimate it is. You only need to know water volume for dosing and maybe water changes.

Is there a better way to calculate water volume aside from just draining every drop out and measuring?

V

Some thoughts: Did you select the correct alk/ca supplement from the pull down menu? Did you make the dosing solution yourself? Are you sure the mixture wasn’t more concentrated than expected?

 

[Kensington Reefer]

Cubic inches divided by 231 gives US gallons

 

[Marcus H]

Cubic inches divided by 231 gives US gallons

I think he's asking what's the actual water held minus sand, rock, etc.

Calcium carbonate density is 2.71g/cm^3 which should mean 22 lb of sand/rock displaces 1g of water.

That's if you have actual reef rock, if you have man made aggregate rock I have no idea. (Probably more dense so more lb needed to displace a gallon)

Regardless, your calcs seem off somehow.

 

[H2OPlayar]

I figure 15-20% is rock and substrate of the total volume.

Really though, for dosing I used to make my own "PID" controller in my mind by adjusting the amount up or down 10% at a time. If I was way off on alk, I would do a one time dose and then maybe up my dosing pump amount 20%.

Now, it is the trident controlled dosing for the win.

 

[Vincerama2]

Some thoughts: Did you select the correct alk/ca supplement from the pull down menu? Did you make the dosing solution yourself? Are you sure the mixture wasn’t more concentrated than expected?

View attachment 43769

Hi Randy. I mixed the BRS Soda Ash, following their instructions... 2 1/3rd cup soda ash with 1 gallon of water.
I pulled down Soda Ash on the product drop down.

Now here's the interesting thing. Having worked out 80g water, I took another Alk reading and got 7.7-8.0 (depending the color of the solution in the Salifert Alk test ... is this intermediate color blue or pink?) So I used their calculator again and plugged in 80g water, initial Alk 7.7, desired alk 9.0, it told me about 75 ml of solution. I added that and after an hour, re-tested the water and came up with 9.0-9.1 alk. So the calculator works when I use 80 gallons of water, instead of my initial guess at 160!

So, having reverse engineered the water volume to 80g, going forward the dosage was almost entirely correct. Now, it could be a fluke, but is the chemistry and math is correct, then it seems that this worked for me. Although my tank is full of rock and sand, it's hard for me to believe that a full 100g (and this includes the 29g sump too) is not water.

NOTE: I do have a running kalk-mixer that does top off, so that would skew the values, since I did also wait a day after dosing to re-measure, since my intent was to get alk to 9.0.
But I'm just surprised that the calculator worked so well after adjusting the water volume.

V

 

[H2OPlayar]

Also keep in mind that Salifert titration kits have a relatively wide error bar, but close enough is all we are aiming for so if it works, keep going.

 

[richiev]

Chiming in from the outside, I think it's much more plausible your solution was higher concentrate than you actually only have 80gal of water in a 180+29 gal tank+sump. That literally means if you pushed all your sand and all your rock into a pile, it would fill over 50% of your tank.

Might be your soda was packed a bit dense and your 1 gallon was a bit low. Might be something else. Just doesn't seem right that you'd have that little water to me.

I personally try and just measure by weight. 1 litter of RODI should weigh 1kg (tare the scale with the bottle on it, and then add). If I can't find weight based measurements of the thing I'm adding, I measure it to volume (eg cups), making sure to keep it fluffy and not packed down, and then weigh that. I then in the future use that weight as my target.

At least then in future mixes I can be pretty confident the strength is consistent, which is likely more important that knowing the exact weight (precision > accuracy).

 

[Vincerama2]

Chiming in from the outside, I think it's much more plausible your solution was higher concentrate than you actually only have 80gal of water in a 180+29 gal tank+sump. That literally means if you pushed all your sand and all your rock into a pile, it would fill over 50% of your tank.

Might be your soda was packed a bit dense and your 1 gallon was a bit low. Might be something else. Just doesn't seem right that you'd have that little water to me.

I personally try and just measure by weight. 1 litter of RODI should weigh 1kg (tare the scale with the bottle on it, and then add). If I can't find weight based measurements of the thing I'm adding, I measure it to volume (eg cups), making sure to keep it fluffy and not packed down, and then weigh that. I then in the future use that weight as my target.

At least then in future mixes I can be pretty confident the strength is consistent, which is likely more important that knowing the exact weight (precision > accuracy).

Yes seems too far off for a 180+29 to only have 80g of water and you are probably right that my mix concentration might be wrong. I’ve not had to readjust as my kalk top off seems to be maintaining alkalinity for the few corals in the tank.

As 80 seems a low guess, I’ll continue to use that when using online calculators and see what happens. Most likely I’ll have to change my estimate until I zero in on a more accurate water volume that works with the calculators to do whatever needs doing. I think it might be that kalkwasser top off skewed my testing and I forgot to turn it off and didn’t measure the immediate all increase (Ie; bad scientific method!)

Oh one other observation is that when I do a large water change, I fill a 44 gallon brute almost to the top, so say 40 gallons and the tank looks like the water is almost half way down. But that’s not saying much as there is exposed rock as well.

I was just thinking that it’s almost impossible to figure out your water volume unless you start with a dry aquascaped tank and count how many gallons of saltwater you actually use to fill it. I didn’t do that, I just filled it with my RO water line and added salt to the not-yet-live tank.

However the method should still work in theory. If you have x amount of water and add y amount of additive then your measurable additive concentration should be z, and you can reverse that equation. With alk, ca and magnesium calculators you should be able to get a good estimate.