Resource Determining ACTUAL tank volume with a water change and a TDS meter

[Naturescapes_Rocco]

Hi all, thought I'd share this useful tip! Many of us are familiar with the estimated volume of our tanks when brand new and empty (or only filled with water); however, the actual volume is dependent on several factors, including the chosen water level, the volume of hardscape, the volume of substrate, and the volume of plants, among others.

My 140-gallon aquarium (journal here!) definitely holds less than the full 140 gallons, but I wanted to know how much it actually holds (in its current state) to help with remineralization, dosing, and the water change % so I can accurately estimate nutrient accumulation, etc. The formula is M1*V1=M2V2, where TDS is the M and the volume is V.

I keep the water level about 1.5" below the rim (held there consistently with RO water Auto Top Off system) to help prevent jumping deaths from newly added fish.

I made sure the water level was fully topped off, then took a large sample of tank water in a jar and set it aside:

Tank water before the water change is about 157 TDS.

I used my old factory-marked rain barrel to remove exactly 70 gallons of water from the aquarium. This is important. The entire situation relies on an accurately measured amount of water removed. I marked this lower water level line with a Sharpie on the side of the tank to always remember how much I need to remove for 70-gallons.

Then, I filled the tank up with RO water. My RO water has a TDS of 1-2, so essentially zero. BEFORE I ADDED ANY REMINERALIZERS/FERTS, I let it flow/cycle with the filter on for about 15 minutes to stabilize the TDS, then took a reading:

Tank water after removing 70-gallons, and adding back 70-gallons of RO water with ~0 TDS, was 47 TDS

You can solve this manually to flex your brain muscles, or... let your brain to waste away by using ChatGPT to solve it faster:



Based on the math, the actual volume of water in my tank is about 100 gallons. This makes sense because the TDS saw a ~70% decrease when adding the 70 gallons of fresh RO water (near zero TDS). There is a lot of massive driftwood, and at least 8 gallons of substrate as well by volume.

You can do this yourself with any water, you just need:

1. An initial TDS reading of your tank's water (keep a sample handy to compare at the end, since TDS meters can fluctuate over time)
2. A known amount of water removed (measured with buckets, pitchers, barrels, whatever. Just make sure it's accurate)
3. A TDS reading of the water you're adding (RO water, tap water, it doesn't matter)
4. Fill the tank to the same level as it was, let it cycle for a few minutes, then take a TDS reading before adding in remineralizers, ferts, food, etc. to keep the TDS stable
5. Place your values in ChatGPT:

"If an unknown volume of water has a TDS concentration of (initial TDS), and I remove a known (known removed gallons) gallons of this water and replace it with a (known removed gallons) with a concentration of (fresh water TDS), and the resulting mixture of the two volumes has a final TDS of (final TDS measurement), what was the initial unknown volume of water?"

So, now you know!

 

[Naturescapes_Rocco]

Determining Actual Water Change % with a TDS meter​

You can also use a TDS meter to determine the % of tank water you're actually changing. This doesn't help you know how many acutal gallons it is, but it does help determine what % of the water is being changed.




The only way to know HOW much water you actually have in your aquarium is by accurately measuring the amount of removed water and doing the initial Volume equation instead of this % equation. Still, this % equation could be helpful for knowing how much water you are actually changing, especially for understanding nutrient accumulation and dilution over time.

 

[JohnPilon]

Holy smokes Rocco this is golden.

Ive been using my first fill as my way to measure it, but sometimes that isnt done, so this could be a LIFESAVER for alot of people.

 

[JuJuFish]

This is AMAZING! Wow. Thanks for sharing!

 

[Pneumonicsonic]

The math is great, and even with just 5 gallons removed and measured you can use this formula. Way to math!

 

[BenB]

I did this differently, but the same using a NO3 test kit, which isn't as accurate, but it worked. I have a Lamotte kit, so it is better than most. Using a TDS meter should be even better.

If you test your TDS, then put in a known mass of KNO3 (or something), give it time to distribute, and then check your TDS, you should be able to back extrapolate your volume. I haven't done the mathing, and I don't have a TDS meter. So, I post this with some hesitation.

 

[ElleDee]

@BenB IIRC, the ppm is that you get from your TDS pen is the concentration of a reference salt required to have the equivalent conductivity as your water. I think my pen uses NaCl. So you would need to know the conductivity of KNO3 or something and then do some converting. The principle still works, but you can't use mass directly... unless you want to dump a bunch of table salt into your aquarium.

 

[BenB]

@BenB IIRC, the ppm is that you get from your TDS pen is the concentration of a reference salt required to have the equivalent conductivity as your water. I think my pen uses NaCl. So you would need to know the conductivity of KNO3 or something and then do some converting. The principle still works, but you can't use mass directly... unless you want to dump a bunch of table salt into your aquarium.

This is beyond more than a basic understanding for me, but I believe you.

I feel like any method is going to induce some degree of error. In the example above, I'm sure each gallon wasn't measured as a 3875ml unit. It just depends on how off we are. Luckily, we don't have to be spot on. When I did it with the KNO3 test kit, it improved my fert dosing a good bit but yeah, still off.

I've grown carnivorous plants in the past, and I had a TDS meter. I'm frustrated that I can't find it. (@ElleDee as a random comment, the water where we live is so weak, that I've watered carnivorous plants with it multiple times with no ill effects. I try to get distilled or rain, but life happens and more than one time, they would get tap. My TDS from the tap was 88 last time I measured it a few years ago.)

 

[Burr740]

Wow. This is so simple and glaringly obvious, wild how its never crossed my mind or even heard it mentioned before

 

[ElleDee]

I've grown carnivorous plants in the past, and I had a TDS meter. I'm frustrated that I can't find it. (@ElleDee as a random comment, the water where we live is so weak, that I've watered carnivorous plants with it multiple times with no ill effects. I try to get distilled or rain, but life happens and more than one time, they would get tap. My TDS from the tap was 88 last time I measured it a few years ago.)

FWIW my tap water has always been between 60 and 90, so it probably hasn't changed recently.

And your point about error is valid, but I think as long as you aim for consistency it doesn't matter if the numbers are off somewhat.

 

[BenB]

I did this today, but I screwed it up somehow despite very careful measuring.
My tank is only a 60p which is supposed to be about 17 gallons. Do you think if a trade out just 1 gallon of water for a gallon of distilled would be enough? I was dreading what I was going to have to do to measure out the water, but when I went to get my 5 gallon bucket, it had graduations on it. That was lucky. Also, if you're thinking about doing this, might be a way for you to measure water also.

 

[Naturescapes_Rocco]

The margin for error is going to be greater the farther away you are from 50% water change for this particular technique. Changing only 1 gallon should work in theory, but your TDS meter (or whatever you're using to compare PPM before and after) might have a hard time determining an accurate difference between the two if only 1 gallon has been replaced. Removing a known, accurate amount of water is technically the most important part of this whole thing, but also by far the most annoying. If you're willing to accurate remove 3.5 gallons in that bucket a few times, you can get a good reading.

 

[BenB]

The margin for error is going to be greater the farther away you are from 50% water change for this particular technique. Changing only 1 gallon should work in theory, but your TDS meter (or whatever you're using to compare PPM before and after) might have a hard time determining an accurate difference between the two if only 1 gallon has been replaced. Removing a known, accurate amount of water is technically the most important part of this whole thing, but also by far the most annoying. If you're willing to accurate remove 3.5 gallons in that bucket a few times, you can get a good reading.

In my test it looked visually like 6-6.5 gallons was about 50%. So I changed 6.5. I messed up somewhere though. I tried to measure accurately. I wonder if stopping and restarting the filter caused it to cough out a bunch of particulate matter with some dissolved stuff as well. Hmmm.. that would cause my volume to appear small though and it was too big.

I'll do it again. My biggest challenge with this bucket is that I was using a short length of tubing to drain the water back into my aquarium and it was difficult to control the flow so as to not dig up plants and gravel. My finger over the opening helped but it only took a small stream in a small tank to go across, hit the aquarium wall, go down, and make a mess. Something to work on.

 

[BenB]

I wanted to follow up. I repeated this test. You can see above that my previous attempt failed miserably.
My tank is small, so there is a much smaller margin of error.
This time I measured out 6 gallons with a gallon jug and replaced it with gallon jugs of distilled water.
As I emptied the water from the tank, I placed it into a graduated 5 gallon bucket to check the accuracy of its scale.
You can see below that the volume I replaced in my previous attempt was probably off by about a gallon which in a ~17 gallon tank is a huge margin of error.