RODI Water: Accurate PH readings and remineralizing - The Planted Tank Forum
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post #1 of 10 (permalink) Old 04-07-2019, 08:46 PM Thread Starter
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RODI Water: Accurate PH readings and remineralizing

My 30 Gal planted tank has been set up for about 7 months now and is doing pretty well. I noticed recently that I'm getting reoccurring cyanobacteria in my tank that I'm guessing is coming from tap water. Also my tap water has low kh and pretty high gh. I had a RODI setup with a 32gal storage tank from my saltwater days so I figured I put it to use on the planted tank after changed all the membranes and filters in the RODI system.


I ordered Nilocg Gh and Kh booster to add needed minerals back into the water. Once I got the gh to ~60ppm or 3.4 deg and kh to ~80ppm or 4.5 degrees I measured the ph (with a PH tester) and it was over 9! From what I'm seeing in searches it hard to get an accurate ph reading on RODI water... but I've added minerals back in. I'm wondering if this is REALLY the ph of my new water or if I'm missing something. Don't want to start adding this water to my tank until I can get a accurate/safe ph reading.


I'm using a VantaKool digital ph meter which is only a few months old. It gives accurate readings of my aquarium water so I'm pretty sure its still in cal. I'm using API 5-in-1 test strips for quick reading of GH and Kh in the RODI water but I do have a liquid test kit that I use regularly on my aquarium. Any help would be appreciated!
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post #2 of 10 (permalink) Old 04-07-2019, 08:57 PM
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I ordered Nilocg Gh and Kh booster to add needed minerals back into the water. Once I got the gh to ~60ppm or 3.4 deg and kh to ~80ppm or 4.5 degrees I measured the ph (with a PH tester) and it was over 9!
How long after you dosed the RO water did you take the reading?

When I dose my RO tanks, right after I add K2CO3 the pH reading shoots up to about 9. Twenty fours later once it has fully dissolved it's 7.05.

May or may not be your situation but possible.
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post #3 of 10 (permalink) Old 04-07-2019, 09:07 PM Thread Starter
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How long after you dosed the RO water did you take the reading?

When I dose my RO tanks, right after I add K2CO3 the pH reading shoots up to about 9. Twenty fours later once it has fully dissolved it's 7.05.

May or may not be your situation but possible.
Great point... I was testing after about 15 minutes. I put a heater and a 550gph powerhead in the RODI storage today after testing and will test again tomorrow night and see what I get. Thanks!
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post #4 of 10 (permalink) Old 04-07-2019, 10:33 PM
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Originally Posted by B.Lace View Post
My 30 Gal planted tank has been set up for about 7 months now and is doing pretty well. I noticed recently that I'm getting reoccurring cyanobacteria in my tank that I'm guessing is coming from tap water. Also my tap water has low kh and pretty high gh. I had a RODI setup with a 32gal storage tank from my saltwater days so I figured I put it to use on the planted tank after changed all the membranes and filters in the RODI system.


I ordered Nilocg Gh and Kh booster to add needed minerals back into the water. Once I got the gh to ~60ppm or 3.4 deg and kh to ~80ppm or 4.5 degrees I measured the ph (with a PH tester) and it was over 9! From what I'm seeing in searches it hard to get an accurate ph reading on RODI water... but I've added minerals back in. I'm wondering if this is REALLY the ph of my new water or if I'm missing something. Don't want to start adding this water to my tank until I can get a accurate/safe ph reading.


I'm using a VantaKool digital ph meter which is only a few months old. It gives accurate readings of my aquarium water so I'm pretty sure its still in cal. I'm using API 5-in-1 test strips for quick reading of GH and Kh in the RODI water but I do have a liquid test kit that I use regularly on my aquarium. Any help would be appreciated!
Strips are not accurate, what is your theoretical gh and kh supposed to be?

Assuming your RODI water has 0 tds and you did the remineralization calculation correctly your storage water should be around ph=7.65 once its mixed properly.
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post #5 of 10 (permalink) Old 04-10-2019, 06:11 PM Thread Starter
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Great point... I was testing after about 15 minutes. I put a heater and a 550gph powerhead in the RODI storage today after testing and will test again tomorrow night and see what I get. Thanks!

I tested my RODI water storage again after 24 hours and low and behold, my PH was at a more manageable 7.7. Thanks for the help! I'm going to use the liquid test kit that I mentioned before to get more accurate reading on KG and GH. As far as what I'm aiming for on these parameters, probably about 5 for each. Any thoughts or suggestions on this would be appreciated!
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post #6 of 10 (permalink) Old 04-10-2019, 08:04 PM
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Yes, this is unsurprising. When you add a concentrated carbonate solution to pure water, which is what you're doing when you add the hardener to your R/O water, it soaks up all the H+ ions as it is partially converted to bicarbonate. This raises the pH fairly spectacularly. It takes time for room air CO2 (or CO2 from injection) to add carbonic acid back to the tank, which restores some of the H+ and lowers the pH again.
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post #7 of 10 (permalink) Old 04-10-2019, 08:12 PM
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Originally Posted by B.Lace View Post
I tested my RODI water storage again after 24 hours and low and behold, my PH was at a more manageable 7.7. Thanks for the help! I'm going to use the liquid test kit that I mentioned before to get more accurate reading on KG and GH. As far as what I'm aiming for on these parameters, probably about 5 for each. Any thoughts or suggestions on this would be appreciated!
Test your TDS of RODI water if its <10ppm you are good, (get a TDS pen). If your pH after 3 days remains stable 7.7(ph meter) you got your kh=5 no need to test it with something less accurate like API liquid test.

You can also test your remineralized water for TDS and match it to theoretical, that is going to be more accurate than the API tests and you will know your kH and gH are dialed in at that point. Much faster than using the API tests.
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post #8 of 10 (permalink) Old 04-11-2019, 10:03 PM Thread Starter
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Test your TDS of RODI water if its <10ppm you are good, (get a TDS pen). If your pH after 3 days remains stable 7.7(ph meter) you got your kh=5 no need to test it with something less accurate like API liquid test.

You can also test your remineralized water for TDS and match it to theoretical, that is going to be more accurate than the API tests and you will know your kH and gH are dialed in at that point. Much faster than using the API tests.

Thanks! I do have a TDS Pen, how would I test the remineralized water's TDS vs "theoretical"? Let's say my TDS is back in the range of 150 TDS (for example, I will test later this evening after work). How would I convert that to a gh and/or kh reading? Also, is it that the API liquid test isn't all that accurate or that any liquid test wouldn't be as accurate as a TDS to gh conversion? I'll gladly spring for a better quality liquid test kit if that is whats needed.


From what I've picked up from multiple searches: 1 degree of hardness should yield between 15-20ppm TDS, depending on what you remineralize with. With this logic, a TDS of 150 could mean between 7.5 and 10 dGH... is this what you're referring to?
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post #9 of 10 (permalink) Old 04-12-2019, 02:16 PM
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Thanks! I do have a TDS Pen, how would I test the remineralized water's TDS vs "theoretical"? Let's say my TDS is back in the range of 150 TDS (for example, I will test later this evening after work). How would I convert that to a gh and/or kh reading? Also, is it that the API liquid test isn't all that accurate or that any liquid test wouldn't be as accurate as a TDS to gh conversion? I'll gladly spring for a better quality liquid test kit if that is whats needed.


From what I've picked up from multiple searches: 1 degree of hardness should yield between 15-20ppm TDS, depending on what you remineralize with. With this logic, a TDS of 150 could mean between 7.5 and 10 dGH... is this what you're referring to?
From @Deanna
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1) Get some, e.g.; KHCO3 (or any fert salt).
2) Use the Rotala Butterfly | Planted Aquarium Nutrient Dosing Calculator calculator and pick a target dKH for one gallon of distilled or RO water. Maybe 8 dKH (easier to measure dKH at higher numbers).
3) For this example, it returns 1.08 grams in 1 gal of pure water = 174 ppm of HCO3 and 111 ppm of K.
4) Add the 1.08 grams to the pure water (assumes your scale is calibrated, which is another story) and make sure it's dissolved well.
5) Sum the ppm numbers, which will return 285 ppm.
6) Stick your TDS pen in the gallon of water and you should get something very close to 204 ppm (if using a 0.5 conversion factor TDS meter - most US TDS meters) or 285 ppm (if using a 0.7 conversion factor TDS meter). Also, take a KH reading to see if you're close to 8 dKH just as triple-check.
---------------------------------------------------------------------------------------------------------------------------------------------------------

Whenever you make up your water instead of testing kH and gH you can just stick in your TDS meter and know how close you are to the theoretical value. I would still test them with the liquid kit the first few times to make sure everything matches and your TDS meter is accurate.
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post #10 of 10 (permalink) Old 04-12-2019, 09:14 PM
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Let me expand a little, after being mentioned by @cl3537:

If you are using a typical US TDS meter, it is based upon a .5 conversion factor (gets into the ec basics of TDS meters) and that means that it will read about 2/3 of actual TDS as ppm. So, if you want to know what the actual ppm is going to reflect on a US-based TDS meter, you have to convert from .5 to .7 conversion factors. You could also buy a .7 conversion factor TDS meter.

A simple way to do this is to use a conversion calculator, such as this: https://www.translatorscafe.com/unit...-conductivity/

To use it, select “parts per million, 700 scale” on the left and “parts per million, 500 scale” on the right. Then put your ppm into the left “From” space and you will see that a US-based (.5 conversion factor) TDS meter will read a lower value than the actual ppm in the sample.

Don’t ask why the US meters do this, unless you can tell me why 2 x 4’s aren’t 2” x 4”.

Regarding increasing the accuracy of the API GH and KH kits, this can be done:

I use 25 ml of test water and divide the number of reagent drops by 5. This will give you a more precise reading of the GH and KH levels.
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Last edited by Deanna; 04-16-2019 at 03:54 AM. Reason: Add
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