Two Indian cities. The same fish. Completely different water. A tank filled in Mumbai and a tank filled in Delhi can behave like two different ecosystems, and the reason is almost always hardness. In Part 2, we saw that pH depends on the water's buffering capacity. Here we look at the numbers behind that buffer.
GH: General Hardness
GH measures dissolved calcium and magnesium. Fish use these minerals for bone, scale and fin health. Shrimp need them to moult. Plants take them up as nutrients. As a rough guide, 0 to 4 dGH is soft, 4 to 8 is medium, and 8 to 12 or more is hard. One dGH is about 17.9 ppm.
KH: Carbonate Hardness
KH measures the carbonates and bicarbonates that act as your pH buffer. High KH keeps pH steady. Low KH lets pH swing. Below roughly 2 to 3 dKH, a tank is at risk of a pH crash. GH and KH are separate numbers. You can have water with moderate GH and very low KH.
TDS: Total Dissolved Solids
TDS is the total of everything dissolved in your water, measured in ppm with a pen meter. It includes minerals, salts and organic compounds, but it cannot tell you which is which. Use it as a trend tool. If TDS climbs steadily between water changes, waste and minerals are accumulating. Very soft supplies need a very low TDS, while hard supplies need a much higher TDS.
Typical Working Ranges
|
Fish group |
GH (dGH) |
KH (dKH) |
TDS (ppm, approx.) |
|---|---|---|---|
|
Guppies, mollies, platies |
8 to 15 |
5 to 12 |
200 to 400 |
|
Tetras and rasboras |
3 to 10 |
2 to 6 |
100 to 250 |
|
Bettas |
4 to 12 |
3 to 8 |
100 to 300 |
|
Goldfish |
6 to 18 |
5 to 12 |
200 to 400 |
|
Neocaridina shrimp |
6 to 10 |
2 to 6 |
150 to 250 |
|
Planted community tank |
4 to 8 |
3 to 5 |
100 to 250 |
If Your Water Is Soft
Soft water is common in parts of western and southern India. The risks are pH crashes from low KH, failed shrimp moults and plants short of calcium and magnesium. Fixes include remineralising gently with GH and KH boosters, adding crushed coral to the filter, or using a natural buffering product like Indica. Raise values slowly and test after every change.
If Your Water Is Hard
Hard water stresses soft-water fish and can make breeding difficult. Two practical options: keep fish that enjoy hard water, such as livebearers and goldfish, or dilute your tap water with RO water to bring numbers down. Water treated by a household salt-based softener is not a substitute. It swaps hardness minerals for sodium, which aquarium fish do not need. We also do not recommend collecting roof rainwater in polluted cities.
Bring It Together
Test your tap water first, then your tank. Choose fish that suit what you have, and only adjust when you must. Keep changes slow. In Part 4, we cover how to test properly with strips and liquid kits.
👉 Shop TCWS Water Test Strips | Shop Sunken Garden Anti-Chlorine | Next: Part 4, How to Test Aquarium Water
