🧪 How to Test GH, KH, and TDS Correctly – Aquarium Water Guide
Water might look clear, but what’s really in it? If you’ve only been testing ammonia, nitrite, and nitrate — you’re missing a huge part of your tank’s chemistry. General Hardness (GH), Carbonate Hardness (KH), and Total Dissolved Solids (TDS) are critical for the long-term success of fish, shrimp, and plants.
This complete guide will walk you through what these parameters mean, how to test them correctly, how to adjust them safely, and what values you should aim for based on your tank type — all in beginner-friendly terms.
🔬 What Are GH, KH, and TDS?
GH (General Hardness): Measures the amount of calcium and magnesium in your water. It affects fish osmoregulation, shrimp molting, and overall mineral availability for plants.
KH (Carbonate Hardness): Measures the water’s buffering capacity. It keeps your pH stable and protects against rapid crashes.
TDS (Total Dissolved Solids): Measures the total amount of dissolved substances in water: minerals, salts, nutrients, and waste. It reflects overall “water density.”
📊 GH vs TDS – What’s the Difference?
This confuses a lot of hobbyists. TDS is a general total, while GH is one specific part of that total:
- ✅ GH = calcium + magnesium only
- ✅ TDS = GH + KH + nutrients + salts + organics + waste
It’s possible to have low GH but high TDS if, for example, you’ve added lots of fertilizers or your tank is dirty.
🧪 How to Test GH
Use a liquid test kit — such as API GH/KH or Salifert GH. Test strips are unreliable for GH. You’ll be counting drops until the water changes color.
- Rinse test tube with tank water.
- Fill to 5 mL line.
- Add 1 drop of GH solution, cap and shake.
- Repeat one drop at a time until color changes from orange to green.
- The number of drops = your GH in °dGH.
🧪 How to Test KH
KH is tested the same way as GH, using the KH bottle from your kit.
- Use 5 mL of tank water in test tube.
- Add 1 drop of KH reagent, shake gently.
- Keep adding 1 drop at a time until the color changes from blue to yellow.
- The number of drops = your KH in °dKH.
🔌 How to Test TDS
You’ll need a digital TDS meter — often called a conductivity or EC meter. These are cheap, fast, and reliable.
- Turn on and calibrate (if needed).
- Rinse the probe in RO or distilled water.
- Dip into tank water and wait for reading to stabilize.
- Record the reading in ppm (parts per million).
📈 Ideal GH, KH, and TDS Ranges by Tank Type
Use this chart to find your target zone. These are time-tested ranges for stability, health, and long-term success.
| Tank Type | GH (°dGH) | KH (°dKH) | TDS (ppm) |
|---|---|---|---|
| Neocaridina Shrimp (Cherry, Blue, etc.) | 6–10 | 3–6 | 200–300 |
| Caridina Shrimp (CRS, CBS, Bees) | 4–6 | 0–2 | 100–160 |
| Betta Tank | 3–8 | 3–5 | 150–250 |
| Discus / Softwater Fish | 1–4 | 0–2 | 80–150 |
| General Community (Tetras, Rasboras) | 4–8 | 2–5 | 150–300 |
| Livebearers (Guppies, Mollies) | 8–12 | 4–8 | 250–500 |
| Low-Tech Planted Tanks | 4–8 | 3–6 | 150–250 |
| High-Tech CO₂ Tanks | 3–6 | 2–4 | 120–200 |
| RO/DI Water (Base) | 0–1 | 0–1 | 0–10 |
🛠️ How to Adjust GH, KH, and TDS
Need to fine-tune your values? Here’s how to safely raise or lower each parameter:
To raise GH:- Seachem Equilibrium
- Crushed coral or aragonite in your filter
- Salty Shrimp GH+ for shrimp tanks
- Crushed coral or baking soda (sparingly)
- Alkaline buffers (Seachem Alkaline Buffer)
- Any additive will raise TDS (salts, ferts, minerals)
- Use RO/DI or distilled water for top-offs or mixing
- Use peat moss or almond leaves to naturally lower hardness
❌ Common Mistakes
- Using expired test kits
- Over-shaking or under-mixing drop tests
- Assuming TDS = GH (it doesn’t)
- Using tap water with unknown mineral content
- Skipping KH and then suffering pH crashes
🔗 Related Water Guides
- How to Cycle a Fish Tank
- Routine Maintenance Tips
- Diagnose Plant Growth Problems
- Water Success Steps for Beginners
📌 Final Thoughts – Test Smarter, Not Harder
GH, KH, and TDS are often overlooked — until something goes wrong. Regular testing takes 5 minutes and can prevent fish stress, shrimp deaths, and unexplained algae or pH swings. Once you know your numbers, you’ll understand your water — and that means you’ll make smarter decisions for your livestock, your plants, and your sanity.
Need help decoding your results? Drop them in the comments and I’ll help you analyze what’s going on in your tank.
