pH and KH stability in aquariums

pH

Measure of acidity or alkalinity

KH / alkalinity

Buffering capacity that resists pH change

Main focus

Trend and stability

Test together

pH, KH, source water and nitrate

Difficulty

Intermediate

Understand the relationship

Aquarium pH is influenced by source water, alkalinity, dissolved carbon dioxide, acids from biological activity, substrate and décor. KH is commonly used by aquarists as a practical indicator of carbonate buffering. Low buffering means normal biological processes can move pH more easily.

Measure source and tank water

Test tap or source water after it has aerated, then compare it with the aquarium. A difference may reveal substrate effects, carbon dioxide, accumulating acids or water-treatment products. Record readings at similar times because planted tanks can show daily pH movement.

Correct causes slowly

If KH is being depleted, review water-change frequency, stocking, feeding and source-water alkalinity. Use remineralisation only with measured targets and consistent preparation. Rapidly raising pH during an ammonia problem can sharply increase toxic un-ionised ammonia.

Common mistakes

• Chasing a species chart with daily pH chemicals.
• Testing pH without checking alkalinity.
• Comparing morning and evening readings as though they were identical conditions.
• Ignoring old-tank accumulation and skipped water changes.
• Changing hardness quickly in a stocked aquarium.

What to record

Record pH, KH or alkalinity, time of day, source-water results, temperature, water changes, carbon dioxide schedule and any new rock, substrate or additives.