Water hardness: Low GH but high KH?

RodgerRodger

I am working on adding a homemade CO2 system to one of my tanks. I was measuring some of the parameters for the water I have available and I am a bit confused with the results. I have 2 water sources, tap water and RO water from a home system. I was checking the GH, KH, and pH to figure out the buffering capacity for each source.

Tap water:
GH = 2 dGH
KH = 13 dKH
pH = 8.4

RO water:
GH = 0-1 dGH
KH = 0-1 dKH
pH = 7.5

For my tap water, I have a very low GH but a very high KH. I don't really understand how this can happen in a water source, or what I can do to address it. I have fish that prefer hard water because I always thought I had hard water, but this would seem to suggest I actually have soft water, it just is also very alkaline.

For my RO water, it doesn't make sense to me for the pH to be so high when the GH and KH are both practically 0. I am using an API liquid test kit, none of the solutions are close to expired or anything, and I double and triple checked all my results. Unless I just got a bad test kit, it should be accurate. I keep my RO water in a 5 gallon bucket with a lid, so it could have to do with dissolved gasses (specifically CO2) being low in the water. That is the only thing I could come up with.

I am most concerned about my GH being really low. There are lots of sources online for raising and lowering KH, but not much for adjusting GH. I have been using half tap water and half RO water for quite a while (because I always thought I had very hard water), but seeing these results is making me question whether that is a good idea, and if I should start using 100% tap water. I know GH is mostly a measure of Ca++ and Mg++ in the water, so if I want to increase it then I would have to increase those 2 ions. I don't know if these are the only 2 elements I need to worry about, or how I could add more of them to my water. Any advise or info is appreciated!
 

LeviS

For calcium some people use crushed coral to up the hardness, either scattered in substrate or in a mesh bag of a filter. My Gh is also soft water but my kh and ph are ridiculously high 300ppm kh and 8.4 ph.
 
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RodgerRodger

From what I've been reading I think the best products for raising GH but not KH are SeaChem Equilibrium, or Wonder Shells. Both provide Ca++ and Mg++ without adding excess carbonates. Another option I found that can be used is a mix of epsom salt (MgSO4) and calcium chloride (CaCl), roughly 1:4 mixture. I was looking at crushed coral, but this adds carbonates to the water along with the calcium, which increases KH as well as GH.
I found this forum post very helpful:
 
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