Lowering aquarium pH safely requires reducing carbonate hardness (KH) - the buffer that holds pH up. Methods that work: 1) Mix RO water 50/50 with tap water - halves KH (and pH). 2) Driftwood (mopani, malaysian, spider) releases tannins that lower pH 0.3-0.5 over weeks. 3) Indian almond (catappa) leaves - shrimp/betta tank favorite. 4) Peat moss in filter - 0.3-0.8 pH drop over 4-6 weeks. 5) Pressurized CO2 in planted tanks drops pH ~1.0. 6) Replace alkaline substrate (crushed coral, aragonite) with inert sand. What NOT to do: chemical "pH Down" (API, Tetra) - doesn't address KH, pH crashes back overnight, kills fish from instability. Phosphate buffers - cloud water, fuel algae. Why pH stability beats target pH: most tropical fish tolerate pH 6.5-8.0 if stable. Wild-caught discus need 5.5-6.8. Apisto species need 5.5-6.5. African cichlids need 7.6-8.6. Test KH first: if KH is 8+ dKH, pH stays high. Lower KH first via RO blend, and pH follows. Safe rate: drop pH no faster than 0.2 per day to avoid pH-shock for established fish.
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