GH, KH and TDS in Aquarium Water: A Simple Guide
GH, KH and TDS in Indian Tap Water: What They Mean (Part 3 of 4)
Mayur Dev|
The water coming out of your tap plays a major role in determining which fish will thrive in your aquarium. This guide breaks down GH, KH and TDS in simple terms, explains suitable target ranges for popular fish, and shows you how to manage Indian tap water when it is too soft or too hard. Welcome to Part 3 of our Water Parameter Series.
Two Indian cities, the same fish, but completely different water conditions. An aquarium filled with water from Mumbai may behave very differently from one filled with water from Delhi, and hardness is often a major reason. In Part 2, we explored how pH is influenced by the water’s buffering capacity. Now, let’s look at the key parameters that help determine that balance.
GH: General Hardness
GH measures the amount of dissolved calcium and magnesium in your aquarium water. Fish rely on these minerals to support healthy bones, scales and fins, while shrimp need them for successful moulting. Plants also use these minerals as nutrients. As a general guide, 0 to 4 dGH is considered soft water, 4 to 8 dGH is medium, and 8 to 12 dGH or higher is hard water. One degree of general hardness (dGH) is approximately 17.9 ppm.
KH: Carbonate Hardness
KH measures the carbonate and bicarbonate content that helps buffer your aquarium water against sudden pH changes. Higher KH generally helps keep pH stable, while low KH makes the water more vulnerable to fluctuations. When KH falls below approximately 2 to 3 dKH, the risk of a pH crash increases. Remember that GH and KH measure different things, so water can have moderate GH while still having very low KH.
TDS: Total Dissolved Solids
TDS represents the combined concentration of dissolved substances in your water, usually measured in parts per million (ppm) using a TDS meter. These substances include minerals, salts and organic compounds, but a TDS reading cannot identify which substances are present. That makes TDS particularly useful for tracking trends over time. If the reading steadily increases between water changes, dissolved waste and minerals may be accumulating. Very soft water supplies generally have lower TDS readings, while hard water supplies tend to have higher readings.
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 |
GH vs KH vs TDS
If Your Water Is Soft
Soft water is found in some parts of western and southern India. However, it can present a few challenges, including pH instability caused by low KH, unsuccessful shrimp moulting, and calcium or magnesium deficiencies in plants. Possible solutions include gradually adding GH and KH remineralisers, placing crushed coral in the filter, or using a natural buffering product such as Indica. Increase the parameters gradually and test your water after each adjustment.
If Your Water Is Hard
Hard water may be unsuitable for fish that prefer softer conditions and can make breeding more challenging for certain species. You have two practical options: choose fish that thrive in harder water, such as livebearers and goldfish, or blend your tap water with RO (reverse osmosis) water to reduce its mineral content. Household water softeners that use salt are not a suitable alternative because they replace hardness minerals with sodium, which is not a desirable way to prepare aquarium water. We also advise against collecting roof rainwater in polluted cities.
Soft vs Hard Water Action Plan
Start by testing your tap water, then test your aquarium to understand how the two compare. Choose fish that are compatible with your existing water conditions, and adjust the parameters only when necessary. Always make changes gradually to avoid stressing your fish. In Part 4, we’ll explain how to test aquarium water accurately using test strips and liquid test kits.
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