Reef Tank Water Parameters: The UK Guide

The ranges at a glance

The table brings together widely used targets. The main reference is Randy Holmes-Farley’s summary of reef water parameters, which compares aquarium targets with natural seawater. Where well-known sources give a different range, we show it.

Ranges from Holmes-Farley (Reefkeeping). Testing intervals are common practice, not rules: test more often in a new tank, while dosing, or when something changes.

The big three: alkalinity, calcium and magnesium

Stony corals and coralline algae build their skeletons from calcium carbonate, so they take alkalinity and calcium out of the water as they grow. Magnesium helps keep calcium and alkalinity from dropping out of solution together. In a tank with growing corals, these three fall over time and need topping up, either by water changes or by dosing.

Alkalinity moves fastest and has the biggest effect, so it’s the one to test most often. Our alkalinity guide covers where to keep it and why a steady level matters. If yours keeps falling, see why alkalinity drops. If you only keep soft corals, consumption is usually much lower, and water changes may be enough to keep up.

Nitrate and phosphate: low, not zero

Nitrate and phosphate come from food and waste. Too much feeds nuisance algae and, in the case of phosphate, can slow coral skeleton growth. Too little can leave corals pale and hungry. Most published targets sit well above zero but far below what you’d see in a fish-only tank.

Sources disagree more here than anywhere else. Red Sea suggests about 2 mg/L nitrate and 0.1 mg/L phosphate for a mixed reef, and far lower for a tank dominated by small-polyp stony corals. Bulk Reef Supply suggests 3–5 mg/L nitrate. Pick a target, then judge it by what your corals and algae do. The nitrate and phosphate guides go through the numbers.

Salinity and temperature: the ones to check first

Salinity drifts up as water evaporates and down if you top up with saltwater by mistake. It’s the first thing to check when something looks wrong, because it changes everything else. Use ppt rather than specific gravity if you can; our salinity guide explains why, and the salinity converter turns one into the other.

Temperature is the easiest to watch and the quickest to cause trouble. A stuck heater or a hot UK summer can push a tank past 28 °C. Keep a second thermometer in the tank so you can spot a failing heater or controller.

Units trip people up

Alkalinity comes in dKH, meq/L or ppm, and they aren’t interchangeable: 8 dKH is 2.86 meq/L or 143 ppm. Phosphate kits may report phosphate (PO₄) or phosphorus (P), and phosphate is about three times the phosphorus figure. Nitrate kits may report nitrate (NO₃) or nitrate-nitrogen (NO₃-N), and nitrate is about 4.4 times the nitrogen figure.

Check the unit on your kit before you compare a reading with any target, including the ones on this page. The free alkalinity converter does the alkalinity sums for you.

Test, write it down, look at the trend

One reading tells you where you are today. Three or four readings over a few weeks tell you which way you’re heading, and that’s what lets you fix a slide before it becomes a crash. Write down the date, the result, the unit and which kit you used. Note anything you changed, such as new corals, a new food or a different salt.

The Companion keeps this log for you. It charts each parameter over 30 days, 90 days or all time, and can remind you when a test is due. It doesn’t judge your readings or tell you what to dose; it just makes the trend easy to see.

What’s the most important reef parameter?

Salinity, temperature and alkalinity are the ones that move fastest and cause the most trouble. Check those first when anything looks wrong.

How often should I test my reef tank?

Weekly for salinity, alkalinity, nitrate and phosphate is a common starting point, with calcium and magnesium every one to two weeks. Test more often in a new tank or while you set up dosing.

Should my parameters match natural seawater?

Natural seawater is a useful reference, but most reef keepers keep alkalinity a little higher and nitrate and phosphate higher than a wild reef. Steady levels matter more than matching the ocean exactly.