The tapwater in my city has GH = 7 and kH = 5. When I do a water change and test the water 1 week later, I find that GH = 10 and kH = 7. What is increasing the hardness of the water?

Here are the specs of the tank:

- 35 gallons
- 4 18W T8 with a piece of mirror used as a reflector
- CO2 injection
- Eheim 2213

I dose KNO3 after every water change to raise NO3 to 10 ppm. I also dose 0.25 ppm Fe, 0.5 PO4 and some amount of K2SO4 every 3 days. I also dose 1 ml of seachem fluorite every other day. The pH is around 6.8 and the temperature is around 27 degrees celsius.

What kind of substrate and rocks do you have?

I'm not really up on this stuff...but isn't flourite a substrate?

KH=carbonates
GH=calcium and/or magnesium

There are only three options -
1. something in your tank is leaching them (most likely - corals, shells, rocks possible culprits)
2. you are adding them (don't know what seachem flourite is, but otherwise nothing you are adding will increase these)
3. you are topping off evaporated water with tap water during the week

If you think something is leaching them, take it out and rinse it off well, put it in a bucket with distilled water. Test GH and KH the next day or a couple days later. If any readings show up, you've got your mystery solved.

It is. He probably meant Seachem Flourish.

The substrate is aquaclay and I have only 1 small piece of driftwood in the tank. Seachem fluorish has some calcium and magnesium in it, but would 1ml every other day account for the increase in GH and kH? It seems very little...

Is this the aquaclay you use: Aquaclay - Ground substrate It appears to be a baked clay product similar to Soilmaster or Flourite. But, I don't know what is in it or if it is intended to buffer the water. It's primary usage seems to be as biological filter media.

Hello VaughnH, that's the product I am using in my tank. Essentially, this seems to be the fluorite without the iron.

Hi, thanks for the info. I have some distilled water left from my DIY fert production and I will do this experiment. Can I assume that distilled water will have a GH and KH of zero?

Distilled water should have extremely low GH and KH, zero in theory, but probably not in ordinarily available distilled water.

Ok, here is how I tested:

- I filled a test tube with tap water and measured the GH and kH. GH was 7 and kH was 6. Then I put aquaclay into this water sample and left it in there for 1 week. Today I have measured the GH and kH again. GH was 8 and kH was 7. So, there is 1 point increase in both. While this explains the increase in the kH that I was seeing, it doesn't explain the increase in the GH, which was becoming 9.