Aquarium pH is driven by carbonate hardness (KH) - the buffering capacity of your water. Why pH is high: 1) High-KH tap water (test KH first - if KH is over 8 dKH, pH stays high). 2) Crushed coral or limestone substrate dissolving (test substrate by dripping vinegar - bubbles = calcium carbonate). 3) Aragonite sand in freshwater (intended for reef). 4) High-KH municipal water source. 5) New tank with degassing - pH appears high until CO2 equilibrates. Safe pH-lowering methods: 1) Mix RO/DI water 50/50 with tap to halve KH (and pH). 2) Add driftwood (releases tannins + tiny acid). 3) Indian almond leaves (catappa) for soft-water tanks. 4) Peat moss in filter (lowers pH 0.3-0.8 over 4-6 weeks). 5) CO2 injection (planted tanks - drops pH ~1.0). 6) Replace alkaline substrate with inert sand. What NOT to do: chemical "pH Down" (API, Tetra) - doesn't address KH, pH crashes back overnight, kills fish. Why pH stability > pH number: most tropical fish tolerate pH 6.5-8.0 if it's stable. Wild-caught discus, apistos, tetras need 6.0-6.8. African cichlids + livebearers thrive at 7.5-8.5. Test: measure pH morning + evening. CO2 from fish respiration drops nighttime pH 0.2-0.4 in planted tanks - normal.
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