Water Chemistry Overview
Aquarium water is not just H₂O — it is a complex solution of dissolved minerals, gases, organic compounds, and ions that profoundly affect fish health. The four primary chemistry parameters — pH, GH, KH, and TDS — describe different aspects of this solution. Understanding them allows you to match your water to your fish's requirements and, more importantly, to maintain the stability that prevents stress and disease.
The cardinal rule of water chemistry is: stability is more important than precision. A fish kept at a stable pH of 7.6 is healthier than one subjected to pH swings between 6.5 and 7.2 caused by well-intentioned but misguided chemical adjustments. Fish adapt to a wide range of parameters over time, but they tolerate rapid changes very poorly.
Understanding pH
pH measures the concentration of hydrogen ions in water on a logarithmic scale from 0 (most acidic) to 14 (most alkaline), with 7.0 being neutral. The logarithmic nature means each whole number change represents a tenfold difference in hydrogen ion concentration — a pH of 6.0 is ten times more acidic than 7.0, and one hundred times more acidic than 8.0.
Most freshwater tropical fish thrive in the pH 6.5–7.5 range. However, species-specific preferences vary significantly:
- Softwater acid-loving species: Discus, cardinal tetras, most South American species — pH 5.5–6.8.
- Neutral-water species: Most community fish, Bettas, guppies — pH 6.5–7.5.
- Hardwater alkaline species: African Rift Lake cichlids, livebearers — pH 7.5–8.5.
Your tap water pH is your baseline. Most municipal water falls between 7.0 and 8.0. Work with your tap water rather than fighting it — choose fish suited to your natural water chemistry whenever possible.
How to Lower pH Safely
- RO water blending: The safest method. Mix reverse osmosis (RO) water (pH ~7.0, zero minerals) with tap water to achieve the desired pH. This simultaneously lowers GH and KH. See the RO water section below.
- Indian almond leaves and driftwood: Release tannins that gently acidify water. Effective for modest reductions (0.2–0.5 pH units). The amber tint mimics natural blackwater habitats. Add 1–2 medium leaves per 10 gallons; replace every 2–3 weeks.
- Peat filtration: Place peat moss in a filter media bag inside the filter. Peat releases humic and tannic acids that lower pH. The effect is gradual and can be controlled by the amount of peat used. Pre-soak peat to remove initial tannin burst.
- CO₂ injection: In planted tanks, CO₂ dissolves to form carbonic acid, lowering pH. This is primarily a plant-growth technique but has a significant pH-lowering effect. CO₂ causes daily pH swings (lower during the day, higher at night) — ensure KH is adequate to prevent crashes.
- Avoid pH-down chemicals: Phosphoric acid-based products (API pH Down) create temporary changes that rebound within hours, causing the very instability you are trying to avoid. They also add phosphate, fuelling algae growth.
How to Raise pH Safely
- Crushed coral: Place in a media bag inside the filter. Coral dissolves slowly in acidic water, releasing calcium carbonate that raises both pH and KH. The effect is self-regulating — it dissolves faster at lower pH and slower at higher pH. Excellent for African cichlid tanks.
- Limestone and Texas holey rock: Decorative rocks that leach calcium carbonate, gently raising pH and hardness.
- Baking soda (sodium bicarbonate): Raises KH (and therefore pH). Dose 1 teaspoon per 20 gallons, dissolved in water, added gradually. Test KH before and after. Do not add large amounts at once — raise KH by no more than 1–2 dKH per day.
- Epsom salt (magnesium sulphate): Raises GH specifically (adds magnesium). Does not directly raise pH but contributes to mineral content. Dose 1 tablespoon per 5 gallons for a 1 dGH increase.
General Hardness (GH)
GH measures the concentration of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These minerals are essential for fish health — calcium supports bone development, scale integrity, and osmoregulation; magnesium is required for enzyme function and nerve transmission. GH is expressed in degrees (dGH) or ppm (1 dGH = 17.86 ppm).
- Soft water: 0–4 dGH. Found in areas with granite or sandstone geology. Preferred by Discus, South American tetras, Bettas, and most rasboras.
- Moderate: 4–12 dGH. The sweet spot for most community tropical fish.
- Hard water: 12–20+ dGH. Found in limestone regions. Preferred by African cichlids, livebearers (guppies, mollies, swordtails), and brackish species.
To lower GH: blend tap water with RO water. To raise GH: add Seachem Equilibrium (balanced calcium/magnesium supplement) or Epsom salt (magnesium only).
Carbonate Hardness (KH)
KH measures dissolved carbonates (CO₃²⁻) and bicarbonates (HCO₃⁻) — the buffering compounds that prevent pH from changing rapidly. KH is the safety net of your aquarium chemistry: adequate KH prevents the pH crashes that kill fish overnight.
The KH-pH Relationship
KH acts as a pH buffer by absorbing and neutralising acids produced by biological processes in the tank (nitrification, respiration, organic decomposition). As long as KH remains above 2 dKH, it can absorb acid production without pH changing significantly. When KH is depleted (approaches 0), pH drops suddenly and dramatically — often from 7.0 to 5.0 overnight — a "pH crash" that is lethal to most fish.
KH is consumed over time as it buffers acid. Water changes replenish it (assuming your tap water has adequate KH). If your tap water is naturally low in KH, supplement with baking soda (1 teaspoon per 20 gallons raises KH by approximately 1 dKH) or crushed coral in the filter.
Minimum safe KH: 2 dKH for most setups. 4+ dKH for planted tanks with CO₂ injection (CO₂ produces carbonic acid that consumes KH). African cichlid tanks should maintain 8–12 dKH.
Total Dissolved Solids (TDS)
TDS measures the total concentration of all dissolved substances in water — minerals, salts, metals, organic compounds — expressed in parts per million (ppm). Measured instantly with an inexpensive electronic TDS meter (pen-style, $10–15).
TDS is a broad indicator rather than a specific diagnostic. It tells you the overall mineral load but not what those minerals are. Its primary uses in aquarium keeping:
- Monitoring RO water remineralisation: Pure RO water has 0 ppm TDS. After adding remineraliser (Seachem Equilibrium or Salty Shrimp GH/KH+), TDS tells you how much mineral content has been restored.
- Detecting gradual accumulation: If TDS rises steadily between water changes despite stable GH and KH, dissolved organic compounds may be accumulating — increase water change volume or frequency.
- Shrimp keeping: Crystal shrimp and Caridina species are extremely sensitive to TDS, requiring 100–200 ppm. TDS monitoring is essential for shrimp breeding tanks.
For most freshwater fish, TDS between 100 and 400 ppm is appropriate. Below 50 ppm stresses most species (insufficient mineral content); above 500 ppm may indicate excessive dissolved waste or overmineralisation.
Using RO Water
Reverse osmosis (RO) water is produced by forcing tap water through a semi-permeable membrane that removes 90–99 % of dissolved minerals, chlorine, chloramine, and contaminants. The result is nearly pure water (0–10 ppm TDS) with neutral pH and zero hardness.
RO water is essential for:
- Keeping softwater species (Discus, wild-type Bettas, South American species) when your tap water is hard or alkaline.
- Precise control of GH and KH through custom remineralisation.
- Eliminating tap water contaminants (phosphate, silicate, heavy metals, nitrate).
Never use pure RO water without remineralising. Zero-mineral water causes osmotic stress — minerals flow out of the fish's body into the mineral-free water, disrupting cellular function. Always add a remineraliser (Seachem Equilibrium for GH, baking soda for KH, or an all-in-one product like Salty Shrimp GH/KH+) to RO water before use.
A typical recipe for softwater tropical fish: blend 75 % RO water with 25 % tap water, or remineralise pure RO to 4–6 dGH and 2–4 dKH. Adjust the ratio to achieve your target parameters.
Species-Specific Requirements
- Discus: pH 5.5–6.8, GH 1–4 dGH, KH 1–3 dKH, TDS 50–150 ppm, temperature 28–30 °C. Soft, acidic, warm water. RO water usually required unless your tap water is naturally soft. See our Discus water parameters guide.
- African Rift Lake Cichlids: pH 7.5–8.5, GH 10–20 dGH, KH 8–14 dKH. Hard, alkaline water. Most tap water requires hardening with crushed coral, limestone, or Seachem Cichlid Lake Salt. See our cichlid water parameters guide.
- Bettas: pH 6.5–7.5, GH 3–12 dGH, KH 2–8 dKH. Adaptable — most tap water is suitable. Indian almond leaves provide mild acidification and antibacterial tannins.
- Arowana: pH 6.0–7.0, GH 2–8 dGH, KH 2–6 dKH. Soft, slightly acidic water. RO blending often needed. Browse our Arowana collection for available specimens.
- Crystal Shrimp (Caridina): pH 5.5–6.5, GH 4–6 dGH, KH 0–1 dKH, TDS 100–200 ppm. Requires buffering substrate (ADA Amazonia) and pure RO water remineralised to exact parameters.
The Stability Principle
Every adjustment you make to water chemistry introduces risk. Chemical buffers rebound. pH adjusters create instability. Additives interact unpredictably. The safest approach to water chemistry is:
- Test your tap water. Know your baseline pH, GH, and KH.
- Choose fish suited to your tap water. This is the path of least resistance and greatest stability.
- If your dream fish requires different water, use RO water blended with tap water to achieve the target. RO blending is the most controllable, predictable method for adjusting chemistry.
- Make changes gradually. Never adjust pH by more than 0.2 units per day or GH/KH by more than 1 dGH/dKH per day. Rapid changes cause osmotic shock and pH shock — both are immediately dangerous.
- Maintain consistency. Use the same water preparation method for every water change. The fish adapt to your specific water chemistry — changing the recipe creates instability.
Water chemistry is not complicated once you understand the principles. Test, understand your water, choose compatible fish, and maintain consistency. The fish will do the rest.