Question Re. Gh and Kh.

[The Pack]

Can anyone explain to me why, in one of my tanks, the GH is testing at 60 mg/L ,but the KH is testing at 130 mg/L. The pH is 7.6 and I have attempted to lower it to less than 7 by replacing some of the water with RO water but the high Carbonate Hardness seems to be frustrating my attempts. I have a gravel substrate, (regular aquarium gravel) , one piece of driftwood, plants and rocks , and some peat in the Aquaclear filter. This is a 35 gal tank. I have avoided any rocks that I thought might contain limestone ,but I'm wondering if I have made an error on some of these and that these rocks may be causing the problem. Any suggestions ?

 

[ronv]

Pack, it sounds like you and I need to swap out some water. See my thread, directly above yours. I think your situation is exactly opposite mine. Your KH is so high, you can't budge your PH. Mine is so low, I can't keep PH stable. The folks on this forum can help you. Good luck.

 

[Randall]

Carbonate-rich water

Hello Pack,

I'll try to address your concerns one by one.

Can anyone explain to me why, in one of my tanks, the GH is testing at 60 mg/L ,but the KH is testing at 130 mg/L. The pH is 7.6 and I have attempted to lower it to less than 7 by replacing some of the water with RO water but the high Carbonate Hardness seems to be frustrating my attempts.

The typical GH test kit measures magnesium levels only, so a reading of 60 mg/L tells you how much magnesium your water contains. We use this measurement to reflect the overall mineral hardness of water, but what we are really measuring is the level of magnesium. Your reading is consistent with minerally soft water.

I have a gravel substrate, (regular aquarium gravel) , one piece of driftwood, plants and rocks , and some peat in the Aquaclear filter. This is a 35 gal tank. I have avoided any rocks that I thought might contain limestone ,but I'm wondering if I have made an error on some of these and that these rocks may be causing the problem. Any suggestions?

The typical KH test kit measures calcium carbonate levels (CaCO3), so a reading of 130 mg/L indicates moderately hard water. However, there are other cabonates in water that your test kit does not measure that also contribute to its overall alkalinity. If your pH is measuring at 7.6, either your water is carbonate-rich out of the tap; or, like you suggest, there is something in the tank that is leaching into the water.

Rocks made of slate, sandstone, petrified wood, or quartz are good for soft water aquaria because they don't leach. Rocks made of limestone, marble, lava, and also shells and corals, will leach CaCO3 into your water and bump up the pH. Have you compared the pH of your tank water with that of your tap water? If the tank water pH is higher than that of your tap water, then something is going on in your tank and can be corrected by removing it. If the pH of your tank water and tap water are the same, then using a tap water/RO water mix should bring the pH down. You will need to play around with the ratios of tap water vs. RO water to achieve the desired results.

Good luck!

Randall Kohn

 

[The Pack]

Thanks for the comments and advice Randall. I have removed all of the rocks which I considered to be questionable and left some sandstone and slate in the tank. I also replaced 20 gal. with RO water. I intend to let things work for a few hours before testing again. By the way would granite rocks present any problem ?

 

[Randall]

Granite rocks

Hello Pack,

Oops! I forgot to include granite on the preferred rock list. Thanks for asking. If the granite is pure, it's okay to use. If in doubt, mix a little acid buffer stuff with a small amount of water and place some on the rock. If the rock bubbles, it fails the acid test and contains something that will leach into water. If there are no bubbles, it's smooth sailing.

Good luck!

Randall Kohn

 

[aspen]

>>'The typical GH test kit measures magnesium levels only.'

actually i believe that most gh kits measure general hardness as the amount of calcium AND magnesium.

>>'The typical KH test kit measures calcium carbonate levels (CaCO3), so a reading of 130 mg/L indicates moderately hard water. However, there are other cabonates in water that your test kit does not measure that also contribute to its overall alkalinity.'

the kh kits i've used are actually alkalinity testers, measuring anything that will contribute to alkalinity. this can be confusing when you have a lower gh reading than your kh reading. this is due to the kh kit measuring phosphates in a water sample which shows up as alkalinity but is not 'carbonate hardness'.

if you go to the various sites which describe their kits i believe that you'll find these things to be the case.

rick

 

[tjudy]

TH (total hardness) is ameasure of both GH and KH.

KH (alkalinity) is the carbonate hardness... which is a measure of the water's ability to resist pH drop.

GH (general hardness) is analogous to conductivity and measures the dissolved solids.

Chemical kits for TH and GH are iffy... No one kit can test both GH and KH. A meter is a better method to test general hardness because the meters test conductivity, and carbonates do not conduct well. As Randall said, most GH chemical test kits measure magnesium, so sodium, potassium or other salts in the water may not be indicated.

A chemical test kit for KH is the only way to go. Lab 'meters' are really colorimeters that more accurately measure the amount of color change in a chemical test. You have to do the chemical reaction, then insert the cuvette into a machine that reads the level of carbonate based upon light defraction.

So... to answer your question... it is possible to have a low GH and a high KH, or vice versa. Normally, however, natural water sources in the USA are either high in both or low in both.

 

[Mike Wise]

Most sandstones are quartz grains cemented together with silica, but not all. Check your sandstone rocks with some acid (scratch the rock with a knife blade & add some acid). If there is a reaction then the cement holding the sand grains in the sandstone together is a carbonate cement & will harden your water.