Hard tap water: how to soften it for your tank

My tap water comes out at 30 °dGH, liquid concrete, basically. If, like me, you live in a hard-water area, you may have watched your plants struggle or given up on shrimp. Good news: you can soften aquarium water cleanly. But before you pour anything in, you have to measure.

The short version. Before softening, measure your GH (total hardness) and KH (carbonate hardness). The most reliable method is to cut tap water with RO (reverse osmosis) water. For a beginner, the simplest option is still to pick fish that suit your water rather than constantly altering it, it’s far less risky.

Hard, soft: what are we talking about?

“Hardness” measures the amount of dissolved minerals, mainly calcium and magnesium. There are two figures:

• GH (general hardness): the total amount of minerals. This is what matters for the health of fish and shrimp;
• KH (carbonate hardness): the water’s ability to stabilise pH. Too low a KH = unstable pH; a high KH = pH that stays high.

“Hard” water has a high GH and KH. Plenty of fish cope with it; others (Caridina shrimp, some tetras and killifish) need soft water.

Step 1: measure first

Never soften blind. A liquid GH/KH test gives you your starting figures and lets you track the effect of each method. Strips will do in a pinch but are less accurate; for adjusting hardness, go for the drops. Here’s our pick:

Step 2: the 4 ways to soften

1. RO water (the most reliable)

A reverse osmosis unit produces water almost free of minerals, which you cut with tap water to hit the hardness you want. For example: half RO, half tap ≈ halves the GH. It’s the method serious aquarists use.

• ✅ Precise, repeatable, ideal for shrimp.
• ❌ The cost of an RO unit, and you have to remineralise pure RO water before use.

No RO unit? You can buy RO water by the bottle from aquatic shops, or sometimes bottled.

2. Peat / catappa leaves

Filtering the water through peat (or adding catappa leaves) releases substances that gently lower KH and pH, tinting the water slightly (a “blackwater” effect). Gentle and natural, but less precise and gradual.

3. Rainwater (with caution)

Naturally soft, but its quality depends on the environment (pollution, roofing). Reserve it for those who can collect it cleanly, and always cut it with tested water.

4. What NOT to do

• The household water softener: it swaps calcium for sodium. The water stays unsuitable and the sodium is harmful.
• Boiling: it acts a little on KH, not on GH, and it’s impractical.

The simplest solution: match the fish to the water

Permanently altering hardness takes discipline. For a first tank, the wisest move is often to leave your water as it is and pick species that like it. In hard water, guppies, platies and mollies are perfectly happy, and you avoid any chemical fiddling.

• I've measured my starting GH and KH
• I know the ideal hardness for the species I'm aiming for
• I've chosen a method (cut RO water, peat…) OR decided to match the species to my water
• I remineralise RO water before using it (never pure)
• I check the GH after mixing, before adding it to the tank

Conclusion

Softening aquarium water means measuring first, then choosing the right method, with cut RO water being the most reliable. But don’t overlook the most relaxing option: picking fish that already love your water. You’ll save yourself a lot of hassle.

To go further, read « Tap water in your aquarium: what to know », and if you’re just starting out, our beginner’s guide lays out every step in order.

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