TDS / KH & GH Reconciliation

My tap water is about 8 dKH and 8 dGH. 8 degrees is about 17.85 ppm. Therefore 8 degrees translates to about 143 ppm. 143 ppm each for KH and GH suggests a minimum of 286 ppm dissolved solids. There is other stuff in the water so I would expect something more than 286.

IMO, home test kits are not that accurate. And I believe the API test kit result of 8 degrees, really means between 7 and 8. If the 8th drop changes the color really fast, then it is closer to 7. If the 8th drop changes the color really slow, then it is closer to 8. So let’s say my water is really only 6 degrees. That would be 107 ppm. Times two is 214. So at a very minimum, I would expect TDS to be mid to low 200s.

My tap water TDS is about 165. Way lower than I expected. So either one or both of my test products is way off or there is something I do not understand about the relationship between TDS and GH and KH.

Is it possible that the GH in my water includes Calcium in the Calcium Carbonate form and it is being picked up by both the KH and GH tests and is therefore being double counted?

I am thinking this is not likely since it is my understanding that Calcium Carbonate does not act as a PH buffer and my water has very stable PH. I tried using API PH down to lower my PH. It took a lot to get it to go down and it did not stay down.

So which is it? Is there something I do not understand? Or are my tests wrong?

AvalancheDave
John58ford
Chanyi

[EDIT]

I just realized that the answer to the question about double counting Calcium Carbonate as GH and KH must be no since I have seen KH drop to near zero during a cycle while GH did not change.

Must be another explanation.

 

TDS measures the dissolved solids that can conduct an electric charge, kH and gH should add to this. If your TDS pen in ppm or is it in μS/m ? Big difference.

Even if your meter is in ppm, it's an adjusted value of conductivity meaning it probably uses a 342 ppm NaCl solution to calibrate itself to - room for error.

That room for error plus room for error in the API kit can add up. It's also not a direct 1 drop = 17.85ppm.

I highly doubt your kH bottomed out from cycling if it was at 8 degrees, but you did state you've used pH down which is probably leading to issues. Ammonia used in cycling also will introduce some weak acids into the tank... scrubbing away at kH and thus dropping pH.

Short answer, no way to tell, don't get hung up on the numbers. If you want to, go with 100% RO water and remineralize it. Trust your math for the numbers, not a test kit.

 

Good answer Chanyi . I will trust my math. I like math... I will also test for reasonableness. I like when things reconcile. And I like to understand why when they do not. GH and KH so far seem to test close to predicted based on dosing RO water.

Even if there is no practical need, I will continue to investigate this as I want to fully understand what my tests are telling me or not telling me.

This leads me to suspect that my TDS meter is less accurate than I hoped. Perhaps so inaccurate as to be useless. My TDS meter has two settings. I am using ppm. I am not sure what the other setting is. I will check. My city water reports GH as 150 ppm. I am sure that varies from time to time and from season to season. But it is close to the 143 that would equate to 8 dGH.

So setting aside the inaccuracy of the tests. I am hearing you say that I am accurately understanding the expected relationship between TDS and GH and KH. If I have 143 ppm of both GH and KH, then TDS should be in the neighborhood of 286. Plus or minus 20% would give me a range of about 230 to 340. So 165 is pretty far off.

To be clear. I never used PH down in any of my tanks. I tested it in bottles of water to see how it would work. It took a lot to move PH and the move was temporary.

KH may not have gotten down to zero, but it got pretty low in a fishless cycle. Low enough that it instantly changed color. Low enough that my PH dropped from 8.2 to the mid sixes. Low enough that my cycle stopped processing ammonia. I added baking soda and the cycle started again. This was done purposefully to see what would happen. Even with my near zero KH, GH levels remained normal. This tells me that the difference is not explained by double counting a part of GH.

The lowest KH got in any of my stocked tanks was 4 or 5 degrees when I went four weeks between water changes due to travel. Other than that both GH and KH have remained very stable with 25% to 50% weekly water changes.

 

Short answer, no way to tell, don't get hung up on the numbers. If you want to, go with 100% RO water and remineralize it. Trust your math for the numbers, not a test kit.
[/QUOTE]
Reminerzlizing ro/di water makes life so much easier. No worries about what day to day changes may occur in your tap water.

 

Yes, I guess you have to balance that with the PIA factor for preparing water change water. I have six tanks totaling 120 gallons. Not a lot by some people’s standards but still 25 to 50% water changes adds up to 30 to 60 gallons of water to prepare.

I suppose once you get your formula down, it is not much more work than dechlorinating.

 

Strange question. Does your meter have "ATD"? (Automatic temperature detection/compensation) The temperature of the water can move your results up and down the scale significantly. I know you are likely testing in 78 ish degree water but allot of the cheap TDS pens don't differentiate.

I use a hm ap-1(still cheapo) and it seems to do well enough in the water temp I use it in but even an oldish (1.3 volts would still be fine in a TV remote this thing starts going bonkers when it's 3 volt batteries get to 2.8) battery dumps the numbers downhill.

I trust my reagent testing much more than the pen and I think I recall you in another post where I described it driving me crazy because I couldn't get the math on my electrically conductive junk to line up. But it's still an ok spot check, just not the right tools for our breed of science.

Which model TDS meter did you choose to procure? I know there are some very nice ones out there but the nicer they are, the more often they need to be calibated it seems.