What your pH reading means
Act todayBelow 5
More acid than any fish in our guide is kept in; test KH and bring it up slowly.
Watch5 to 6
Low: fine for soft-water fish such as cardinals and discus; filter bacteria slow down below about 6.
Good6 to 8.5
The usual range. It is wider than the 6.5 to 7.8 most community fish share because soft-water fish sit near its low end and rift-lake cichlids near its top, so check it against each fish’s care page.
Watch8.5 to 9
High for most community fish; suits rift-lake cichlids. Keep ammonia at 0.
Act todayAbove 9
Too alkaline for any fish in our guide; look for what is raising it.
For a tank with fish. The same bands run in the test reading checker, which reads all your results together.
What pH measures
pH is how acid or alkaline the water is, on a scale where 7 is neutral, lower is acid and higher is alkaline. The scale is logarithmic: pH 6 is ten times as acid as pH 7, so a change of one whole number is a big change for fish.
Why the pH drifts down
Filter bacteria use up carbonate as they turn ammonia into nitrate, and fish waste adds acid, so the pH slowly falls between water changes. While there is KH in the water it holds the pH steady; once the KH runs out, the pH can drop suddenly, which the Merck Veterinary Manual calls old tank syndrome. Topping up evaporated water without changing any makes it worse. KH explained
pH and ammonia
The higher the pH, the more of any ammonia is in its toxic form. Going from pH 7 to pH 8 makes about ten times as much of the same reading toxic. Keep ammonia at 0 in hard, alkaline tanks. The ammonia table
Match the fish to the water
Each care page gives its fish’s pH range. Livebearers, rainbowfish and rift-lake cichlids like it alkaline; tetras, rasboras, discus and rams like it acid. Picking fish for the water from your tap is easier than changing the water every week. Fish for hard water · Fish for soft water
Raising or lowering it
To raise pH and keep it steady, raise KH: crushed coral or aragonite in the filter or substrate dissolves slowly. To lower it, mix tap water with RO or distilled water, or add driftwood, peat or dried leaves, which release tannins. Injected CO₂ lowers pH during the hours it runs. Whatever you use, change it over days and test before and after; a pH that swings stresses fish more than one that sits a little outside their range. Test at the same time of day, since plants and CO₂ move the pH between morning and evening. Check all your readings together