How much water do you really have in your tank? HINT: it's not what you think.

[Biglew11]

Any way of calculating how much water our tanks really hold? Formulas?

length x width x height of the inside dimensions divided by 231, it's kind of a guess what your rocks displace

i have a 75 gallon tank with about 20 gallons in sump. my total water volume is about 85 gallons is pretty close

 

[Zionas]

Thanks. Sounds good.

 

[piranhaman00]

I calculated by with real math but since we dont have science grade instruments its a good approx.

I used alkalinity so I could check between a few different test kits. With tank full and running (with no inhabitants) test alk. With known solution of dkh solution dose a known amount of liquid. Test the water again. Using that increase in dkh one can determine the exact volume of tank. Its not exact but works for me.

 

[Dr. Jim]

It is very important to know how much water is in one's system. This information is essential for dosing additives or meds. Whenever I start up a new tank I will carefully measure how much saltwater I add. If the tank isn't huge, I will use a bucket where I drew a line on the outside at the 4 gal mark. (This level was determined by adding water from a trusted 1 gallon container). If the tank is large, I will measure the amount of water in the mixing tank with marks drawn at the level of every 4 gallons, measured with the bucket described above.

 

[ca1ore]

1. How much water volume do you think you actually have in your main display and what was the manufacturers advertised gallons or liters?

I had a thread years ago noting that most tank manufacturers state volume based on external dimensions when it really should be based on internal dimensions. My current tank, for example is a 450 based on OD but only 400 based on ID. 50 gallons of acrylic being the difference.

I wonder how many people that stated their system volume in this thread did it the right way LOL. Oh and displacement by rocks and sand further reduces net water volume.

2. How important is it to have an accurate measurement of water volume in your system?

There were two or three responses to my thread, which I concluded meant most people didn't care. In retrospect, they were probably right .... a distinction without a difference. Very few things you will do with your tank require a highly precise measure of volume. I'm sure there will be folks that think that knowing precise volume is important in dosing meds .... until they take a moment to read the % error on their hobby-grade test kits.

 

[Trever]

I got surprised.

Figured I'd have at least 42 gallons and possibly more like 45, and I calculated that by including the various factors like water level, rock/sand, etc.

But the actual water I added when filling was 38 gallons.

This was disappointing and I'm glad I put the largest possible sump in my small stand.

One factor I omitted was inside vs. outside tank dimensions, which adds up, even though I have thin (6 mm) glass on the DT.

I assume that rock and sand also displaced significantly more than I anticipated.

The only other factor was the water in the live sand. In that sense, I may have a little more water than I think I do (39 gallons?), impossible to know.

 

[Willy315]

Any way of calculating how much water our tanks really hold? Formulas?

http://reef.diesyst.com/volcalc/volcalc.html

 

[Xjv152]

The tank i have is sold as 180 gallon but after rock was added holds 140 gallon. 20 gallon refugium

 

[reefruss]

it's kind of a guess what your rocks displace

I recently did a complete tear down to get rid of nuisance algae and I put all the rock into a second tank after I cleaned it. I marked the water level before putting the rock in and then calculated the volume displaced by the rock. I have, what I would consider, a lot of rock in my tank but was surprised to calculate it only to be 8 gallons of displaced water. That's for approximately 110# of rock in a 90 gallon display.

 

[dbowman5]

i voted not very important on the basis of the "accurate" portion of the question.
it is important to have an approximation of the volume. it is important to consistently use that approximation. when making additions make 80% of the calculated addition. if you are worried retest after a few hours and add a percentage of the add based on the difference in the effect of the add compared to the expected effect.
Knowing the actual volume or weight of additions is as important as knowing the volume of the tank.
EX.:
i think i have 100 gallons. i want to raise a constituent 10 percent. i add 8%; i retest and find i raised it 4%; i realize my tank is 200 gal. i recalculate based on 200 gal and add 4%; it raises the concentration to a total of 8%;
from then on i can add 10% of 200 gal.
but by adding smaller adds i don't overshoot until i know the effect of the actual add.
fyi, if a certain level is the sweet spot and there is a consistent drop of that value, i would try to add a little extra on a regular basis. otherwise the average concentration will always be below optimum.

 

[K7BMG]

For me I feel its important to know. Not just for dosing as that is just an initial measurement and from there it changes.

It matters most on an ethics level.

 

[themcfreak]

I voted not that important. All of our things we dose are measured with hobby grade test kits. All of our medicine, even in an empty quarantine tank with a few PVC pipes, are estimations. I am not measuring exactly 4.2858 ml of a medicine to get exactly 2.0 ppm. I dose 4, measure, and if my hobby grade says 2, I am happy. If it says 1.8, I might add half a ml and see what that does

Even with dosing alk/calcium. At first, you will dose, and if you continuously get higher than expected (or lower) because you dosed based on having a 100 gallon tank, when you only have 80, then you adjust your dose and continue from there. In the end, it works itself out to a best of average.

I have an 85 gallon tank and 39 gallon sump. Anytime I use the calculators, or try to estimate dosing, I use the 100 gallon mark as where I start from. Then, go from there

 

[madweazl]

150g display and 62g sump. Total water volume is as close to 150g as I can determine. Finding the volume is actually pretty easy to figure out by dosing alkalinity. I guesstimated original volume and then dosed alkalinity to desired level. After measuring the results, I was able to figure out actual water volume using a dosing calculator.

Outside of dosing, the actual volume is meaningless to me.

 

[thermoJoe]

So, when the tang police and similar say you need a 100 gal tank for this or that fish, are they referring to the actual volume or the inflated one?

 

[ca1ore]

Any way of calculating how much water our tanks really hold? Formulas?

Measure INTERNAL dimensions - length in inches x depth in inches x height in inches divided by 231 = gallons. Cannot help if you live your life on the metric system. Won't help with displacement though.

 

[K7BMG]

I have a 180 that was custome made for a person that did not pick it up.
So I got a deal on the tank.
Though called a 180 the dimentions were smaller than the standard 180. I knew this going in.
My calculated volume is actually 142G

 

[ca1ore]

So, when the tang police and similar say you need a 100 gal tank for this or that fish, are they referring to the actual volume or the inflated one?

Guidelines aren't that precise LOL

 

[Zionas]

No problem, I can do these conversions easily lol.

 

[glb]

When I filled my 40g breeder and 20g tall sump for the first time, with all the rocks, sand, and equipment in place, it came to 47g total. I’m really glad I have that initial measurement. I realize over time that corals will grow and displace water, but that’s over a long period of time. It’s very helpful when dosing. That being said, consistency is more important than precise numbers. If you don’t know the exact amount of water in your tank, estimate the best you can and then stick to that number at first. If it’s off, you’ll realize it over time, eg if you use a dosing calculator for alk and the alk is lower than it should be, your estimated total water volume is probably lower than you thought. Just keep an eye on things and make adjustments slowly. Just my 0.02.

 

[RoweReef]

I have an RSR 350 with sump; 60lbs of rock and 40lbs sand. Subtracting 20% of gross water comes to ~75 gallons. The 20% is a guess. 75 gallons seems close.