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Got a new test kit in today to test both General Hardness- GH (permanent hardness) and Carbonate Hardness - KH (temporary hardness.

I cannot for the life of me figure out what would be making my tank water so much harder than the tap water I'm adding. Any suggestions?

Here is all the data you should need.

Tap Water:
GH - 84ppm
KH - 50ppm

Planted Tank:
GH - 250ppm
KH - 180ppm

Substrate: Pea Gravel on top of Organic potting soil with Vermiculite mixed in.

Filter: Fluval 306 with PhosGuard (I had initial brown algae issues) and activated carbon

Injected CO2 with diffuser (off topic, but I really want a reactor)

See the picture for contents. I have a large piece of driftwood and a rock (that doesn't look or feel calcium based to me, but maybe it is) holding it down while it soaks up water.
 

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Take the rock out and test it in a bucket of water.
Put it in pure tap water, test GH, KH, pH (and TDS if you have a meter) when you start, then test the next day, then every few days for a week.

That rock sure looks like the culprit to me!

However, test a handful or two of gravel at the same time. (separate container, of course).
If you have any of the potting soil left over, test that too. (in a 3rd container) Some potting soils have calcium carbonate in some form added to keep the pH from dropping too low.
Might be beneficial to test pure tap water with nothing in it, too. (4th container)
 

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That would be the famous Texas holey rock, I believe! That is certainly a soft form of limestone and will raise your readings if you have soft, acidic water. Sells for more than $1 a pound if you choose to change it out.
 

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Discussion Starter #5
Done!

Take the rock out and test it in a bucket of water.
Put it in pure tap water, test GH, KH, pH (and TDS if you have a meter) when you start, then test the next day, then every few days for a week.

That rock sure looks like the culprit to me!

However, test a handful or two of gravel at the same time. (separate container, of course).
If you have any of the potting soil left over, test that too. (in a 3rd container) Some potting soils have calcium carbonate in some form added to keep the pH from dropping too low.
Might be beneficial to test pure tap water with nothing in it, too. (4th container)
Thanks for the suggestion, Diana! I'm now holding down the driftwood with a piece of slate and I've placed the rock in a bucket with tap water. I'll test over the next few days and let you know! If it turns out the rock isn't the culprit, I'll proceed to test the gravel and soil.

I appreciate the idea. You must be a developer with such excellent debugging skills!
 
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