Why pH, KH, and GH Matter in Aquariums
If you’ve ever had fish die mysteriously after a water change or plants suddenly melt — unstable water hardness might be the cause. pH, KH, and GH are deeply connected, and understanding them helps you build a stable, thriving tank.
This guide breaks down what each parameter means, how to test for it, and how to adjust it without harming your ecosystem.
⚖️ What Is pH?
pH measures how acidic or alkaline your water is — on a scale from 0 to 14.
- pH 7.0: Neutral
- pH < 7.0: Acidic (soft water)
- pH > 7.0: Alkaline (hard water)
Most freshwater tropical fish thrive in a pH between 6.5 and 7.8. However, stability is more important than hitting a specific number.
🧱 What Is KH (Carbonate Hardness)?
KH is your water’s buffering capacity — it resists changes in pH. Low KH means your pH can swing rapidly, stressing or killing fish. High KH means your pH stays more stable.
- Low KH: < 3 dKH — fragile, easy to crash
- Moderate KH: 4–8 dKH — ideal for most community tanks
- High KH: 10+ dKH — common in African cichlid or tap water tanks
If your tank suffers from mysterious pH drops, it’s likely a buffering problem — not a pH problem.
💧 What Is GH (General Hardness)?
GH measures the amount of dissolved minerals — mostly calcium and magnesium — in your water. It affects osmoregulation (how fish and inverts manage internal water balance), molting in shrimp, and plant growth.
- Low GH: < 4 dGH — soft water; good for tetras, discus, shrimp
- Moderate GH: 4–8 dGH — ideal for most planted tanks
- High GH: 10+ dGH — better for livebearers and cichlids
🧪 How to Test pH, KH, and GH
You’ll need a liquid test kit that includes KH and GH drops (API or JBL are popular options).
- pH: Test weekly, especially after water changes
- KH: Test monthly or when pH fluctuates
- GH: Test monthly or when breeding inverts or specific fish
Test your tap water, too — this tells you your baseline before adjustments.
🔧 How to Adjust KH and GH Safely
- To increase KH: Add crushed coral, aragonite sand, or baking soda (¼ tsp per 10 gal)
- To decrease KH: Use RO/DI water or mix with distilled water
- To increase GH: Add Seachem Equilibrium, Wonder Shell, or mineralized root tabs
- To decrease GH: Use RO water or specialized resins (not common)
Always adjust slowly — test in between each change and avoid large shifts in a single day.
🐟 What Fish Prefer Soft vs. Hard Water?
- Soft water fish: Discus, tetras, dwarf cichlids, ram cichlids, freshwater shrimp
- Hard water fish: Guppies, mollies, platies, African cichlids, snails
💡 Final Thoughts
pH, KH, and GH can sound intimidating — but they’re just different ways of measuring stability and mineral content. Keep them within your fish’s comfort zone, and you’ll avoid mysterious deaths, algae blooms, and pH crashes.
Not sure where to start? Head over to our Water Testing Guide or troubleshoot your tank in our Water Quality Fix Guide.
