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TDS / GH / KH query

seb tries to scape

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Joined
2 Jan 2021
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7
Location
East of England
Hi all, my tap water has GH 22 and KH 15 (API liquid test kit) and my TDS pen (generic from Amazon) gives a reading of 347. I'm beginning to use RO water for a new nano scape I'm setting up for a Betta and I'm trying to work out how to safely and consistently re-mineralise with tap water.

I'm aiming for a soft water setup, with a KH around 3 (although I'm using Seiryu stone in the tank, so expect the KH to rise between water changes.)

Basically, I'm not too clued up on water chemistry, but I see lots of advice to remineralise water to 120 - 150 TDS. However, my RO product water has a TDS of 22 and when I add tap water to remineralise to KH 3 (GH 6), the resulting water only has a TDS of 86, which seems low, when API's dKH to PPM conversion rate indicates GH 6 = 107.4PPM.

How can my TDS result be lower than the GH reading alone (TDS results are PPM right)? Is my TDS pen faulty? Am I missing something? Am I worrying over nothing?

I'm happy to be educated. My aim is just to provide stable, soft water, consistently moving forward.

This is my first post, but looking forward to being a part of this great community.
 

X3NiTH

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13 Apr 2014
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1,364
Hi there and welcome!
TDS pen (generic from Amazon)
Is my TDS pen faulty?

Out of 4 generic HM TDS3 pen types that I’ve had only 2 gave an accurate result. The first died after a year or so of use, the LCD went kaput. Bought same type again and the unit couldn’t read anything lower than 200TDS after I had recalibrated down from 2,000 (this is trying to read 0TDS RO/DI, the lower the conductivity the harder it is to read, there are no instructions to recalibrate you have to work out the secret key combination to get it into that mode), out of the replacement two for that replacement (doubling my chances of getting a good one) one was DOA and the other luckily is still fairly accurate after a few years (within at least 1-2% of the calculated total last time I checked), they’re generally poorly assembled, I find that the potting compound used to isolate the electrics from water ingress is hit or miss, I’ve dismantled 4 now so I guess that kind of makes me an expert opinion (ingress is what did in the LCD of the first unit, the leak was tiny so done in by evaporating moisture), they are ok if you get an accurate working one.

You can buy known conductivity calibration liquids to test the accuracy of your device and if you can work out the key combo you could recalibrate the device if necessary.

You pretty much get what you pay for.

Eyballing colour change in a test kit can sometimes leave room for errors, just a general statement, not calling you colour blind!

If you are not adding supplemental co2 the stone shouldn’t give you too many problems with rising dKH as long as the water remains neutral or above.

:)
 

sparkyweasel

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30 Jun 2011
Messages
2,140
Welcome! :)
I would suspect the TDS pen too, especially as it reads your RO water at 22ppm.
 

seb tries to scape

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2 Jan 2021
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Cheers guys 👍

Sounds like the TDS pen is misreading then (thanks for your detailed insight X3NiTH.) Especially as when I first bought it, it gave me a reading of 478 from my tap, which at that time was GH was 20 and KH 10, so I was a little surprised the TDS had gone down when GH and KH had gone up.

Eyballing colour change in a test kit can sometimes leave room for errors
Fortunately with both the GH and KH tests, you add 1 drop of solution to 5ml of water, shake and repeat the process until the water changes colour from either green to yellow for GH, or blue to yellow for KH, so assuming the test kit is accurate, I'm pretty confident with this one 😛

I'll stick with my plan of remineralising to KH3, which is basically cutting RO with 20% tap water then.

If you are not adding supplemental co2
I am injecting CO2...
 

Nick potts

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25 Sep 2014
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Torbay
I'd pick up some calibration solution, just to check your tds meter, HM Digital TDS and EC Calibration Solution 342ppm

Are you making your own RO?

I remin to 120tds but don't check the parameters, so don't know the starting KH, but i did test the other day before a WC and the tank was at 5KH (and it is full of Seiryu and co2 injected)
 

Wookii

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Nottingham
Cheers guys 👍

Sounds like the TDS pen is misreading then (thanks for your detailed insight X3NiTH.) Especially as when I first bought it, it gave me a reading of 478 from my tap, which at that time was GH was 20 and KH 10, so I was a little surprised the TDS had gone down when GH and KH had gone up.


Fortunately with both the GH and KH tests, you add 1 drop of solution to 5ml of water, shake and repeat the process until the water changes colour from either green to yellow for GH, or blue to yellow for KH, so assuming the test kit is accurate, I'm pretty confident with this one 😛

I'll stick with my plan of remineralising to KH3, which is basically cutting RO with 20% tap water then.


I am injecting CO2...

You might find you don’t need to remineralise with anything other than some Mg in your macro dosing mix, depending on how much Seiryu stone you have in your tank and how low the CO2 drives your pH.

I use pure non-reminerlised RO in my tank which has a load of Seiryu stone in it. I water change 25% daily and can’t get the KH below 6 or the GH below 11.
 

seb tries to scape

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East of England
I think I'll either recalibrate my pen, or just buy a new, better quality one.

Are you making your own RO?
Yes, I've got a Green Aqua 50GPD unit that I've got connected up in the garage and storing in a 120L container. In hindsight, should've got 200GPD based on current throughput!

I remin to 120tds but don't check the parameters, so don't know the starting KH, but i did test the other day before a WC and the tank was at 5KH (and it is full of Seiryu and co2 injected)
I use pure non-reminerlised RO in my tank which has a load of Seiryu stone in it. I water change 25% daily and can’t get the KH below 6 or the GH below 11.
Interesting, I don't have a huge amount of Seiryu, I didn't weigh it but I'd guess under 5KG (I'll attach a hardscape photo.) That's quite a variance in results in both your setups. I'll do some tests on mine in a few days to see where it is on entry and at water change. I only planted it Friday evening and still getting everything dialled in.

Hardscape.jpg
 

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