Water hardness, comparing different test kits

[Jess]

I guess this sort-of follows Cory's video from 3 years ago comparing API Freshwater Master Test Kit and Tetra 7-in-1 strips.

I bought the API GH & KH tests, too.  My tank has been set up for almost a month.  I'm a scientist, so I test anything and everything (my spreadsheet has 58 lines of test results over a 26-day period).  I have consistently found that the API and Tetra tests of water hardness are inconsistent. 

• API: GH = 0-1 degree (0-17 ppm), KH = 2 degrees (17-36 ppm) 
• Tetra: GH = 50 ppm, KH = 0 ppm. 

Does anyone know why they are so inconsistent?  They're basically opposite (I have triple-checked that I'm looking at them correctly).

I'm not seeing crazy pH swings in my tank. It's a 17-gallon, running pressurized CO2 at 1-3bps, with ADA Amazonia aquasoil.  pH runs 6.2-6.4 consistently.  The lowest I've ever measured it is 6.0, and the highest is 6.6.  Most of the plants are still adjusting, but the most interesting case is my Pogostemon helferi.  I bought a tissue culture and a potted version and both arrived in awesome condition; the former has completely melted over 3 weeks and the latter is slowly petering out since it arrived about 2 weeks ago. I let it acclimate in its pot, and just planted it in the substrate today. I suspect it will die. Cory mentioned once that P. helferi needs hard water. (Darn. I really love that stupid plant.)  Other plants seem ok so far: Eleocharis vivipara, Eleocharis 'Belem' (dwarf hair grass), Ludwigia arcuata, buce and anubias, H. pinnatifida, A. reineckii 'Mini', dwarf baby tears (tissue culture version had no roots, Cory's potted version seems ok though).  I dose Easy Green (2 pumps once a week), but I am considering adding Seachem Equilibrium (might buy potassium & calcium test kits first, if I can find them).

So I guess it is safe to say my water is "very soft" and I don't need to know the exact level...but I just wonder why the two different tests are so different and whether anyone has experienced a similar observation. 

[Jess]

Update: I found a detailed water quality report for my county.

• Alkalinity = 21 ppm
• General hardness = 30 ppm 
• Calcium = 9.4 ppm
• Magnesium = 2.6 ppm
• Potassium = not detected
• Iron = not detected

The different scales for the two tests make it a bit tricky to come to conclusions, but I feel more comfortable now.  

I remember when I first kept a tank I did SO MUCH tinkering with the water parameters; all because of all the internet panic about low GH and KH.  As a wiser adult, I've just been a lot more hands-off. That is (so far) working much better!