How to lower or raise pH in an aquarium, safely

DiagnosticTank is months old, water changes are infrequent, KH tests at 2 dKH or below.

FixRestore buffering rather than dosing pH-up. Aquarium Co-Op warns against letting KH fall to 2 dKH or below, recommends 4–8 dKH for most tanks, and suggests about 1 lb of crushed coral per 10 gallons in the substrate or a bag of it in the filter, replaced every 6–12 months. Regular water changes also restore KH. See the low KH page.

DiagnosticpH climbs after adding limestone-based rock, crushed coral or dolomite.

FixRemove or reduce the calcareous material; Aqueon says these dissolve slowly and raise and buffer pH. Mix in RO or deionised water gradually to bring hardness and pH down.

DiagnosticYou keep or plan to breed fish from soft, acidic water.

FixAqueon's methods: RO or DI water (tested before use), aquarium-grade peat in a mesh bag in the filter, or driftwood, noting that one or two small pieces won't do much in a large or well-buffered tank. Aquarium Co-Op suggests letting the tank drift down with fewer water changes and gradually mixing in RODI water. Change pH before adding fish, or slowly if they're already in.

DiagnosticpH is lower first thing in the morning than in late afternoon; heavily planted or algae-heavy tank.

FixNormal to a point: the NRAC water chemistry fact sheet explains that CO2 from respiration lowers pH overnight and photosynthesis raises it by day, and that higher alkalinity means smaller swings. Raise KH if the swing is large.