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Water Hardness Calculator

Convert calcium and magnesium ion concentrations to total water hardness expressed as CaCO₃. Classify hardness and compare against species-specific recommended ranges.

Aquatic Facility ScienceCaCO₃Client-Side

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Load example water hardness data to see the full workflow

Ion Concentrations

Source: Westerfield, The Zebrafish Book

  • Converting Ca²⁺ and Mg²⁺ test results to total hardness as CaCO₃
  • Checking whether your water hardness is appropriate for your species
  • Planning RO/DI dilution to reach a target hardness
  • Monitoring Ca:Mg balance in recirculating systems

Don't use for

  • As a substitute for direct water testing — always measure ion concentrations with a validated kit
  • For carbonate hardness (KH) — this calculator measures general hardness (GH) only
  • For precise marine reef chemistry — use a full reef chemistry panel instead

Water Hardness Chemistry

Water hardness is calculated by converting individual ion concentrations to their CaCO₃ equivalents:

Calcium hardness = Ca²⁺ (mg/L) ×\times 2.497 Magnesium hardness = Mg²⁺ (mg/L) ×\times 4.118 Total hardness = Calcium hardness + Magnesium hardness
  • Ca factor: MW(CaCO₃) / MW(Ca) = 100.09 / 40.08 = 2.497
  • Mg factor: MW(CaCO₃) / MW(Mg) = 100.09 / 24.31 = 4.118

A healthy Ca:Mg ratio for freshwater is approximately 3:1. Significant deviations may indicate unusual source water or contamination.

Species-Specific Hardness Requirements

Different aquatic species have evolved for different water chemistries:

  • Zebrafish (Danio rerio): 75–200 mg/L — native to moderately hard streams in South Asia
  • General freshwater fish: 50–250 mg/L — broad tolerance for most tropical species
  • African cichlids: 160–320 mg/L — require harder, more alkaline water
  • Marine/reef: 350–450 mg/L — high calcium is critical for coral calcification

Sudden changes in hardness are more stressful than gradual adjustments. When modifying hardness, change by no more than 50 mg/L per day.

Frequently Asked Questions

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