Guide 26 — Water Chemistry Guide

Water Testing 101

You can't manage what you don't measure. This guide covers what to actually test for, the difference between liquid kits and test strips, and how often testing genuinely needs to happen.

The Core Parameters

Most day-to-day problems in an aquarium trace back to one of a small handful of measurable parameters. Testing isn't about chasing perfect numbers — it's about catching a drifting trend before it becomes a crisis.

Parameter	What It Tells You
Ammonia (NH₃/NH₄⁺)	Waste product from fish and decay. Should read zero in a cycled tank — any reading is an early warning.
Nitrite (NO₂⁻)	Intermediate stage of the nitrogen cycle. Also toxic — should read zero once cycled.
Nitrate (NO₃⁻)	End product of the cycle. Not immediately toxic at low levels but builds up between water changes — your main long-term maintenance indicator.
pH	Acidity/alkalinity. Stability matters more than hitting an exact number.
GH & KH	General and carbonate hardness — mineral content and buffering capacity. Covered in full in our GH & KH guide.

Liquid Test Kits vs Test Strips

Both formats work by triggering a colour-change reaction to a specific compound in the water, then comparing that colour against a reference chart. The difference is in the method and, generally, the precision.

Liquid reagent kits — such as the widely used API Freshwater Master Test Kit, or ranges from JBL, NT Labs and Salifert — use drops of liquid reagent added to a water sample in a test tube. They're generally regarded across the hobby as the more precise and consistent option, and are the format most experienced keepers default to, particularly for ammonia and nitrite.

Test strips are dipped directly into the tank and read against a colour chart within a set time. They're faster and more convenient, which makes them useful for a quick daily check, but the colour bands are typically broader and reading them can be more subjective. Strips are also more sensitive to storage conditions — humidity and age degrade them faster than liquid reagents in sealed bottles.

How Often to Test

Testing frequency depends on where the tank is in its life. During cycling, daily or every-other-day testing of ammonia and nitrite is standard — this is how you track the cycle progressing. Once established and stable, weekly testing of nitrate, pH and hardness is generally enough to catch a slow drift.

Test immediately, regardless of your usual schedule, whenever something changes: after adding new stock, following any sign of stress or unusual behaviour, after a large water change, or if algae growth or cloudiness appears without an obvious cause.