Beyond ammonia, nitrite, and nitrate, four additional parameters determine water quality: pH, GH (general hardness), KH (carbonate hardness), and TDS (total dissolved solids). Mastering these allows you to replicate the natural habitats of any fish species and prevent pH crashes.
pH: Acidity vs. Alkalinity
pH measures hydrogen ion concentration on a 0-14 scale (7 = neutral, <7 acidic, >7 alkaline). Most tropical fish tolerate 6.5-7.5, but specialized fish require extremes: discus and cardinal tetras prefer pH 5.5-6.5, while African cichlids and livebearers need pH 7.5-8.5. The cardinal rule: Stable pH is more important than a "perfect" number. Rapid pH swings (more than 0.3 in 24 hours) stress fish fatally. To lower pH naturally: driftwood, peat moss, Indian almond leaves, or CO2 injection. To raise pH: crushed coral, aragonite, baking soda (temporarily).
GH: General Hardness (Calcium & Magnesium)
GH measures the concentration of calcium (Ca²⁺) and magnesium (Mg²⁺) ions. Soft water (0-8 dGH) is essential for Amazonian fish (tetras, angelfish, discus) and many plants. Hard water (12-20+ dGH) suits African cichlids, guppies, mollies, and goldfish. GH affects osmoregulation — fish in wrong hardness levels suffer kidney stress and poor breeding. Adjust GH using remineralizing salts (for RO water) or dilution with RO water to reduce hardness.
KH: Carbonate Hardness (Alkalinity)
KH measures bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions. Its primary function is pH buffering — preventing rapid pH drops. Low KH (0-4 dKH) leads to pH crashes (sudden drops to pH 5 or below), which kills fish overnight. High KH (8-15 dKH) keeps pH stable and alkaline. For planted tanks and soft-water fish, maintain KH 3-6 dKH to provide buffering without raising pH too much. For African cichlids, target 8-12 dKH. Increase KH with baking soda (1 tsp per 20 gallons raises KH by about 4 dKH) or crushed coral substrate. Decrease KH by diluting with RO or distilled water.
TDS: Total Dissolved Solids
TDS measures all dissolved organic and inorganic substances (salts, minerals, waste products). Measured in ppm (parts per million). Most freshwater aquariums run 150-400 ppm. Discus and wild-caught fish need very low TDS (50-150 ppm). African cichlids and brackish tanks may have 400-800+ ppm. High TDS from waste indicates poor water quality — do a water change. Low TDS water (RO/DI) needs remineralization for fish health. TDS meters are inexpensive and useful for monitoring water change effectiveness.
How to Test and Adjust Safely
Use liquid test kits (API, Salifert) for accurate pH, GH, KH. TDS meters are electronic. Never adjust parameters by more than 10-15% per day. Acclimation: Drip acclimate new fish for 1-2 hours when parameters differ significantly from your tank. When changing water, match temperature and treat with dechlorinator before adding. For sensitive species (discus, shrimp), use RO/DI water remineralized to target values.
Common Problems & Solutions
pH crash: Low KH (<2 dKH) allows pH to drop rapidly. Add baking soda or crushed coral to buffer.
Hard water for soft-water fish: Dilute tap water with RO water or use peat filtration.
Soft water for hard-water fish: Add crushed coral to filter or substrate, use remineralizing salts.
Rapid pH change after water change: Age and aerate new water overnight, or use a pH stabilizer product.