pH Level

pH is logarithmic — pH 6 is ten times more acidic than pH 7. Most freshwater aquarium fish tolerate pH 6.0–8.0, but breeding is often only triggered within a species' native range: Discus breed optimally at pH 6.0–6.5 while African cichlids require pH 7.8–8.5. Marine aquariums must maintain pH 8.1–8.3 for coral health — reef acidification below pH 7.8 dissolves carbonate skeletons. In planted aquariums, CO₂ injection lowers pH during the day; degassing at night allows it to rise — this natural fluctuation is well-tolerated by most fish. Water changes using tap water with a different pH can cause rapid, harmful fluctuations if not pre-treated.