Guide
Aquarium water hardness: KH and GH
GH tells you how much calcium and magnesium the water holds, KH how many carbonates keep the pH steady. Both are measured in degrees of German hardness (°dH).
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GH and KH: the difference
- GH (general hardness): calcium and magnesium. It affects fish health, shrimp moulting and plant growth.
- KH (carbonate hardness): the buffer that stops pH from dropping sharply. From KH and pH you can also work out CO2 concentration.
1 °dH equals about 17.8 mg/l (ppm) of calcium carbonate (CaCO3).
What hardness for which fish
| Water | GH | Example fish and animals |
|---|---|---|
| Soft | 2–8 °dH | neon tetras, discus, most Amazon fish |
| Medium | 8–15 °dH | most captive-bred fish, corydoras, Neocaridina shrimp |
| Hard | 15–25 °dH | guppies, mollies, African lake cichlids |
Low KH and pH crashes
With KH below about 2 °dH the pH can drop suddenly, especially with CO2 injection and lots of decaying matter. A sharp pH drop stresses fish and slows the bacteria of the nitrogen cycle. If your water is very soft, check KH regularly.
How to change hardness
- Lower it: mix tap water with reverse osmosis (RO) water. Half and half gives roughly half the hardness.
- Raise it: a remineraliser for RO water, or limestone gravel such as dolomite in the filter; it raises KH and GH.
- Change it gradually over several water changes, not in one day.
Your local water supplier usually publishes the hardness of tap water on its website.
How Aquarium Monitor helps
The app records KH and GH test results with dates, reads test strips from a photo once calibrated against the colour chart from the pack, and works out CO2 from KH and pH.
Frequently asked questions
How do I measure KH and GH?
Liquid drop tests are the most accurate: you count drops until the colour changes. Strips are quicker but less precise.
Is tap water fine for an aquarium?
Usually yes, with a conditioner. Check its hardness and compare it with what your fish need.
Keep an eye on your tank from your phone
The app is free and ad-free. The monitor runs on your Raspberry Pi.