Partial water change & targeted softening - Tools & Calculators

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Partial water change & targeted softening

Calculate the mixing ratio of tap water and RO/soft water during a partial water change to reach your target value.

How do I soften aquarium water for sensitive fish and plants?

Why soften aquarium water?

Many popular aquarium species originate from soft, acidic waters – think South American tetras, discus, Apistogramma, and most aquatic plants. Hard tap water can stress these species, inhibit breeding, and limit plant growth. By mixing tap water with RO (reverse osmosis) or distilled water, you can achieve the ideal water parameters for your livestock.

Understanding GH and KH

  • GH (General Hardness): Measures calcium and magnesium ions. Affects fish osmoregulation and plant nutrient uptake.
  • KH (Carbonate Hardness): Measures bicarbonates. Acts as a pH buffer – low KH allows pH swings, very important for CO₂ systems.

Target values for common setups

Setup type GH KH
Soft water fish (tetras, discus)3–8 dGH1–4 dKH
Caridina shrimp4–6 dGH0–2 dKH
High-tech planted tank4–8 dGH2–5 dKH
General community6–12 dGH3–8 dKH
African cichlids10–20 dGH10–18 dKH

Methods for softening water

  • RO water mixing: The most reliable method – mix RO (0 GH/KH) with tap water to reach your target
  • Rainwater: Free and soft, but may contain pollutants – filter before use
  • Peat filtration: Lowers KH and pH naturally, adds tannins (amber color)
  • Active substrate: Some aqua soils buffer to low pH and consume KH

The mixing formula

RO ratio = (Tap value − Target value) ÷ Tap value

Example: Tap water KH 10, target KH 4. RO ratio = (10 − 4) ÷ 10 = 0.6, so use 60%% RO and 40%% tap water.

Tips for water softening

  • Make changes gradually – sudden hardness shifts stress fish
  • Pure RO water lacks essential minerals – remineralize for shrimp and sensitive species
  • Test both tap and tank water regularly to track changes
  • Prepare water changes in advance to ensure consistent parameters