Reef Tank Parameter Checker
Enter a test result and see whether it is low, in range, or high, how far it is from the ideal, and how fast you can safely correct it. Covers alkalinity, calcium, magnesium, salinity, temperature, pH, nitrate, and phosphate.
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ideal range
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gap to ideal
Important: this is a safe parameter checker, not a precise dosing tool. The exact amount of any additive depends on the specific product and your real water volume, and the wrong dose is dangerous. For exact milliliters, always use your product's official instructions or a trusted dosing calculator such as the BRS reef calculator. Correct slowly, and never chase a number with a big swing, since fast changes kill corals.
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How to Read Your Reef Parameters
A successful reef tank is really a stable-chemistry tank. Corals and invertebrates evolved in the open ocean, where water chemistry barely changes, so the goal of testing is not to hit a perfect number but to hold a steady one. The checker above compares your reading to the accepted ideal and acceptable ranges, tells you whether you are low, in range, or high, and shows how far you are from the nearest ideal bound. From there it gives you a safe correction ceiling, because how fast you change a value matters just as much as the value itself.
The Big Three: Alkalinity, Calcium, and Magnesium
Most daily reef chemistry comes down to three linked values. Alkalinity, measured in dKH, buffers pH and supplies the carbonates corals use to build skeleton; most reefers hold it between 8 and 9 dKH, with 7 to 11 acceptable. Calcium pairs with carbonate in coral skeleton and sits best at 400 to 450 ppm. Magnesium, ideally 1300 to 1350 ppm, is the balancer that keeps calcium and alkalinity in solution. These three move together as corals grow, so if one is stubborn, check the other two. A common rule is to fix magnesium first, since low magnesium makes alkalinity and calcium nearly impossible to hold.
Why Slow Corrections Matter
The fastest way to lose corals is not a value that is slightly off, it is a value that swings. Even a change toward the ideal is a shock if it happens too quickly. Safe limits most reefers follow are no more than about 1 dKH of alkalinity per day, roughly 50 ppm of calcium per day, and about 100 ppm of magnesium per day. If a reading is far out of range, spread the correction over several days and retest between doses rather than dumping a large correction at once. Small, steady adjustments give corals time to adapt and protect the rest of the tank.
Salinity, Temperature, pH, and Nutrients
Beyond the big three, a few supporting parameters round out a healthy reef. Salinity should sit at natural seawater levels, specific gravity 1.025 to 1.026, confirmed with a calibrated refractometer. Temperature is best held steady between 76 and 78 degrees Fahrenheit, with 75 to 80 acceptable. pH usually runs 8.1 to 8.4 and tracks alkalinity and tank carbon dioxide. Nutrients are not the enemy: a small amount of nitrate (1 to 10 ppm) and phosphate (0.03 to 0.10 ppm) actually feeds corals, while zero can cause pale, starved tissue. As always, stability and slow change beat chasing any single perfect number.
This Is a Checker, Not a Dosing Calculator
One more time, because it matters: this tool tells you where you stand and how fast you can safely move, but it deliberately does not tell you how many milliliters of a product to add. Every additive is concentrated differently, and the right dose depends on your real water volume, so the only safe source for an exact amount is the product's own instructions or a trusted calculator such as the BRS reef calculator. Use this page to plan the direction and pace of a correction, then dose with your product's numbers.
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Frequently Asked Questions
What is the single most important reef parameter?
Alkalinity stability is widely considered the most important parameter to manage day to day. Alkalinity (carbonate hardness) buffers pH and supplies the carbonates corals use to build skeleton, and it is consumed fastest, so it swings first. Most reefers hold it between 8 and 9 dKH. The exact number matters less than keeping it steady, since a swinging alkalinity is one of the most common causes of tissue recession and coral loss, especially in small-polyp stony corals.
Why do reef parameters have to be corrected slowly?
Corals adapt to their water chemistry, and a fast change is a shock even when it moves toward the ideal. Raising alkalinity by more than about 1 dKH per day, calcium by more than roughly 50 ppm per day, or magnesium by more than about 100 ppm per day can burn coral tissue and trigger rapid tissue necrosis. Big swings, up or down, kill corals faster than a value that is simply a little off. When in doubt, split a correction over several days and retest between doses.
How are alkalinity, calcium, and magnesium related?
These three move together and are often called the big three. Corals pull calcium and carbonate (alkalinity) from the water in a roughly fixed ratio to build their skeletons, so the two drop in tandem as corals grow. Magnesium is the balancer: it keeps calcium and alkalinity in solution and prevents them from precipitating out as they would in plain saltwater. If your magnesium is low, alkalinity and calcium become very hard to hold steady, so always check and correct magnesium first.
What causes alkalinity to drop in a reef tank?
The main driver is coral growth: a healthy, growing reef consumes carbonates continuously, so alkalinity falls a little every day. Heavy coralline algae growth, clams, and other calcifiers add to the draw. Other factors include the type of salt mix, the amount of livestock, and any chemical filtration. This is why mature reefs need ongoing dosing or a calcium reactor to replace what is consumed. Track your daily consumption over a week to size your dosing routine.
How often should I test my reef water?
When a tank is new or you are dialing in a dosing routine, test alkalinity every day or two, plus calcium and magnesium weekly. Once the system is stable, many reefers test alkalinity twice a week and the rest of the big three weekly, along with periodic nitrate and phosphate checks. Salinity and temperature should be confirmed regularly. Consistency matters more than frequency: testing at the same time of day with the same kit gives you trends you can actually act on.
Does this tool tell me exactly how much to dose?
No, and that is by design. This is a safe parameter checker that tells you whether a reading is low, in range, or high, and how fast you can correct it without harming livestock. The exact milliliters of any additive depend entirely on which product you use and your real water volume, and dosing the wrong amount is dangerous. For exact dosing, always use your specific product instructions, the bottle calculator, or a trusted tool like the BRS reef calculator.