Reef aquarium water parameters: what to test first

Which reef aquarium water parameters should you check first when corals open less fully or a test result suddenly changes? A long list alone is not much help. What matters is interpreting measurements and keeping changes traceable. Our suggested routine: check the basics first, then review consumption and nutrients, and investigate other causes systematically.

1. Check temperature and salinity first

Record temperature and salinity together with the time. Check whether the top-up system, heater and water level appear normal too. This helps you avoid overlooking a simple cause while already considering additives. If readings conflict, check the measuring device according to its instructions and repeat the measurement.

Salinity, density and specific gravity describe different quantities. Compare units and methods rather than numbers alone. Our salinity and calibration guide explains how to approach this.

2. Track alkalinity, calcium and magnesium over time

Enter alkalinity, calcium and magnesium in a simple log. Include water changes and adjustments to your supplementation. One reading describes the current position; several comparable readings help reveal changes. For interpreting alkalinity trends, see our guide to stable reef aquarium alkalinity.

If a value falls despite unchanged dosing, first check that the dosing pump is actually delivering: inspect the reservoir, tubing and settings. If the equipment works and repeated readings confirm the trend, consumption may have increased. Calculate an adjustment using the actual water volume and the concentration of your dosing product. A reading from another aquarium does not help with that calculation.

Relevant tests in our shop include the Salifert KH/ALK Test and Tropic Marin Ca/Mg Combitest Professional. Choose a test for the parameter you actually want to measure.

3. Record nitrate and phosphate together

Log nitrate and phosphate alongside feeding, livestock and changes to filtration or adsorbers. Compare readings taken under similar conditions where possible. An unexpected result is a reason to check again; it is not yet a reason to start several interventions at once.

Record the parameter and unit: phosphorus in ppb is not the same numerical value as phosphate in mg/l. Directly measured orthophosphate and phosphate calculated from phosphorus are not automatically comparable either. Check the method in the laboratory report before trying to correct an apparent difference from a home test.

For these two parameters, browse the Salifert Nitrate NO3 Test and Salifert Phosphate PO4 Test.

If a phosphate test reads zero or gives an unexpected result, our guide to measuring phosphate in a reef tank helps you check the detection limit, units and comparison with laboratory analysis.

4. Confirm unusual results before correcting them

• Check the instructions, expiry dates and storage of reagents.
• Check sample volume, clean containers and the specified reading time.
• Repeat a surprising measurement carefully. Use a suitable reference or a second method if needed.
• Record a confirmed deviation, then decide on a specific response.

Changing everything at once makes it difficult to identify what helped afterwards. This sequence is a guide for routine checks, not a diagnosis for every closed coral.

5. When does ICP analysis complement home water tests?

A laboratory report is particularly useful when you have a specific question that home tests cannot answer, such as whether an unwanted metal is present or a trace element is unusually low. Check the kit’s parameter list. If alkalinity or salinity is unusual today, confirm it at home rather than waiting for a posted sample to be processed.

Our comparison of ICP analysis and home water tests helps you choose a method for your question. Browse our reef aquarium water tests when selecting equipment.

Browse our dedicated ICP analysis category. Options include Fauna Marin Reef ICP and Reef ICP Test Total; choose according to the analysis scope you need.

A short checklist for your testing log

Date and time · parameter and unit · test used · result · water change or dosing adjustment · observations of livestock. Keep entries simple enough that you will continue making them.

If the pumps stop during a power outage, prioritise water movement and gas exchange. Our reef aquarium power-outage guide takes you through the first steps and restarting the equipment.

Frequently asked questions

How often should I test reef aquarium water?

After changing dosing, check the affected parameter more frequently at comparable times until you understand the trend again. In a stable aquarium, choose regular testing days and keep a record. Test outside that routine if equipment fails, livestock behaves unusually or a water change produces unexpected readings.

Are good water readings enough when a coral stays closed?

Also consider placement, lighting, flow and possible contact with other animals. A testing log helps narrow down the possibilities, but does not replace observing the coral itself.

If nitrate readings disagree, check the method first: Our guide to nitrate testing and comparing NO3 results walks you through the main checks.

A yellow tint or unwanted dissolved substances are a different question from alkalinity or nutrient readings. Our guide to activated carbon in a reef aquarium explains when the filter medium is useful, how to use it and what it cannot replace.