API 5-in-1 Test Strips

API 5-in-1 Test Strips give you a quick read on five water values from a single strip: pH, nitrite, nitrate, carbonate hardness (KH) and general hardness (GH). You dip a strip in tank water, wait, and compare the pads to the color chart on the box. API makes them for both freshwater and saltwater aquariums.

What the five readings tell you

• pH. Shows whether the water is acidic or alkaline. Sudden swings matter more than the exact number.
• Nitrite. Should read zero in a cycled tank. Any nitrite usually means the biofilter is still building or has been disturbed.
• Nitrate. Builds up between water changes. A rising number is your cue to change more water or feed less.
• KH. Carbonate hardness buffers pH. Low KH lets pH crash.
• GH. General hardness, which matters for fish and shrimp that prefer soft or hard water.

What it doesn’t measure

There is no ammonia pad on this strip. In a new tank, or after fish deaths, you’ll want ammonia too. A liquid kit such as the API Freshwater Master Test Kit covers ammonia and gives finer readings. Reef keepers also need alkalinity, calcium and salinity tests that strips don’t replace, like the Salifert alkalinity kit.

Getting a reliable reading

Use dry hands and close the container right after taking a strip, since moisture can ruin the rest. Hold the strip still in the water for the time given in the directions, then read it in good daylight. Compare all five pads before the colors keep changing. API suggests weekly checks, plus extra checks whenever the water looks off or fish act strangely.

Box sizes

This page covers the 25-count box. The same strips also come in a 4-count box, handy as a trial or an emergency spare, and a 100-count box for keepers with several tanks. The strips and readings are the same in every size.

Who should look elsewhere

If you need precise numbers to chase a problem, or you’re cycling a tank and need ammonia, a liquid kit is the better tool. Strips are best as a fast weekly check.