“Hard water” and “soft water” get thrown around a lot in fishkeeping, and it can be confusing. For guppies, the key takeaway is that they usually do best in stable, mineral-rich water. That doesn’t mean you need extreme hardness — it means you need enough buffering and minerals for consistent pH and healthy development. If your local supply is already moderate (common in many areas), you can often keep guppies easily with basic dechlorination and steady maintenance.
How to tell what you have
- GH (general hardness): measures minerals like calcium and magnesium.
- KH (carbonate hardness): measures buffering that helps stabilise pH.
- pH stability: soft water often swings more between changes.
Signs your water might be too soft for guppies
- pH drifts downward over time
- KH tests very low
- fish seem “touchy” after water changes due to instability
Gentle ways to improve stability
- Raise KH slowly: crushed coral in the filter can add buffer over days.
- Consistent water changes: smaller, steady changes reduce swings.
- Don’t chase pH: stability matters more than a specific target.
Hard vs soft water is less about labels and more about stability. If your KH is strong enough to prevent swings, guppies usually settle, breed, and hold colour much better.
