About Stock: Reef Supplement Dosing Basics for Calcium, Alkalinity and Trace Elements

Every hard coral in a reef tank is quietly pulling calcium, carbonate and magnesium out of the water to build its skeleton. In a wild reef that supply is endless. In a glass box it runs out, and when it does, growth stalls, tissue recedes at the base, and polyps stop extending properly. Supplement dosing is simply the job of replacing what the corals removed, at roughly the rate they removed it.

The three-part concept

Calcium, alkalinity and magnesium are usually talked about as a trio because they interact chemically. Alkalinity (measured as dKH) is the water's buffering capacity and its supply of carbonate ions. Calcium is the other half of the calcium carbonate that corals lay down. Magnesium acts as a stabiliser, it keeps calcium and carbonate from precipitating out of solution together as a useless white haze on your rockwork and pump impellers.

Push calcium up without alkalinity and the two will fight each other, since raising one tends to pull the other down as corals consume both. Let magnesium drift low (healthy reef range is roughly 1250 to 1350 ppm) and you will find both calcium and alkalinity become unstable and hard to hold, no matter how carefully you dose. This is why experienced keepers test all three together rather than chasing one number in isolation.

Typical target ranges

• Calcium: 400 to 450 ppm
• Alkalinity: 7 to 9 dKH, held steady rather than swinging
• Magnesium: 1250 to 1350 ppm

Stability matters more than hitting an exact figure. A tank sitting steadily at 7.5 dKH will generally outperform one that bounces between 6 and 10 through inconsistent dosing.

Matching a supplement line to your tank

Small mixed reef tanks with a modest coral load can often get by on manual liquid dosing. A product such as Coral Essentials Calcium 500ml or a general-purpose Complete Calcium supplement is straightforward to measure with a syringe and add by hand once or twice a day, and it suits tanks where consumption is low enough that daily demand does not fluctuate wildly.

As stony coral coverage increases, so does the daily draw on calcium and alkalinity, and manual dosing from a jug becomes harder to keep consistent. This is where two-part or three-part systems designed for heavier SPS and mixed reef systems earn their place, ideally run through a dosing pump on a controller so additions happen in small amounts spread across the day rather than one large daily slug that spikes pH and alkalinity.

Trace elements and colour

Calcium, alkalinity and magnesium cover the bulk building blocks, but corals also draw on a long list of trace elements, iodine, strontium, manganese, iron and others, that influence colouration and general vigour rather than skeletal growth. Seachem Reef Iodide targets iodine specifically, which supports coloration and can be depleted faster in heavily skimmed or carbon-dosed systems. Broader trace blends such as Red Sea Trace Colors A 500ml are formulated to replace a spread of these minor elements at once, useful for keepers who do not want to run a shelf of single-element bottles.

How to actually get this right

Test before you dose, not after. A basic alkalinity, calcium and magnesium test kit run weekly will tell you your tank's actual consumption rate, and that number should drive how much you add, not a generic bottle instruction. Increase doses gradually, no more than about 1 dKH or 20 ppm calcium per day, since rapid swings stress coral tissue more than a slightly low but stable parameter ever will.

Because product ranges and pack sizes shift with supplier availability, the most reliable way to see what dosing lines and trace supplements we currently carry is to check out our stock list, which is updated as new stock lands and older lines sell through. Pairing a supplement range with a decent test kit, browse the Hanna checker range for fast alkalinity and calcium readings, will do more for coral health than any single bottle on its own.

For keepers wanting a full ecosystem approach rather than mixing individual elements, the Aquaforest range is worth a look, and for automating the whole process the Seachem lineup pairs well with basic dosing setups for smaller systems.