Aquarium Water Chemistry Explained (pH, KH, GH, TDS)
If you’ve ever felt lost staring at a water test strip or trying to understand what GH or KH even means—you’re not alone. But getting your water chemistry right is the foundation for a thriving, stress-free aquarium.
This guide breaks down pH, KH, GH, and TDS in plain English—so you can test, adjust, and maintain your tank with confidence.
Table of Contents
- What Is pH?
- What Is KH (Carbonate Hardness)?
- What Is GH (General Hardness)?
- What Is TDS (Total Dissolved Solids)?
- How to Test Water Parameters
- Ideal Ranges for Freshwater Aquariums
- How to Adjust pH, KH, GH, and TDS
What Is pH?
pH measures how acidic or alkaline your water is, on a scale of 0 to 14. A pH of 7 is neutral. Below 7 = acidic, above 7 = alkaline.
- Low pH (Acidic): Can stress or kill alkaline-preferring species
- High pH (Alkaline): Can burn gills of acid-loving species
Pro Tip: Don’t chase “perfect” pH. Consistency is more important than hitting an exact number.
What Is KH (Carbonate Hardness)?
KH is your water’s ability to buffer or resist changes in pH. It stabilizes your pH so it doesn’t crash or spike suddenly, which is deadly for fish.
- Low KH: Unstable pH, prone to crashes
- High KH: Very stable pH, harder to lower
What Is GH (General Hardness)?
GH measures the amount of calcium and magnesium ions in the water. It affects fish bone health, osmoregulation, and plant growth.
- Low GH: Soft water, ideal for tetras, discus, shrimp
- High GH: Hard water, preferred by livebearers like guppies, mollies
What Is TDS (Total Dissolved Solids)?
TDS is the total amount of minerals, salts, and organic matter dissolved in your water. Measured in ppm (parts per million), it’s a quick snapshot of water quality.
Important: TDS is not a parameter you adjust directly, but a result of GH, KH, fertilizers, and waste buildup.
How to Test Water Parameters
- Test strips: Fast but less accurate
- Liquid test kits: More accurate, good for KH/GH/pH
- TDS meter: Digital tool to get ppm reading in seconds
Test weekly when starting a tank, and monthly once stable. Always test before water changes or adding new livestock.
Ideal Ranges for Freshwater Aquariums
| Parameter | Ideal Range | Notes |
|---|---|---|
| pH | 6.5 – 7.5 | Match to species, not universal |
| KH | 3 – 8 dKH | 5+ dKH preferred for stability |
| GH | 4 – 12 dGH | 8+ dGH for livebearers |
| TDS | 100 – 400 ppm | Varies by tank type |
How to Adjust pH, KH, GH, and TDS
- To raise KH: Add crushed coral, baking soda, or alkalinity buffer
- To lower KH: Use RO/DI water or peat moss
- To raise GH: Use Seachem Equilibrium or Wonder Shells
- To lower GH: Mix tap with RO water
- To lower pH: Driftwood, peat, almond leaves (slow but natural)
- To raise pH: Crushed coral or pH buffers
Always make changes slowly to avoid shocking your fish. Monitor fish behavior closely when adjusting parameters.
Final Thoughts
Water chemistry isn’t just for advanced hobbyists—it’s for everyone who wants healthy fish and thriving plants. Test regularly, adjust slowly, and prioritize stability over perfection.
Need help interpreting your test results? Drop a comment below or check out our beginner’s guide to aquarium maintenance here.
