Dialing In Your Reef Tank's Water Parameters (Without Overcomplicating It)

Most aquarium hobbyists who dip their toes into the coral market and see any success will soon be stocking their aquariums with hundreds of dollars in corals. These corals, specifically those of the large-polyp and small-polyp stony coral families, need very specific parameters to ensure their survival. While some people overcomplicate this — leading to many misconceptions among hobbyists — replicating these parameters is quite simple and can be achieved by a variety of methods. This article looks specifically at the water parameters a thriving home reef aquarium needs, and the methods you can use to achieve them.

The Trio That Actually Builds Your Coral's Skeleton

If you only ever learn three numbers, make it these — calcium, alkalinity, and magnesium, because they don't really work independently of each other.

Your corals are pulling calcium and bicarbonate straight out of the water column to build their skeletons, which is essentially just calcium carbonate. The actual building happens in a thin layer of specialized cells called the calicoblastic epithelium, tucked between the coral's living tissue and its existing skeleton, where the coral concentrates calcium and carbonate far above what's in the surrounding water and precipitates it out as aragonite, a specific crystal structure of calcium carbonate. That whole process is powered largely by energy the coral gets from its symbiotic algae's photosynthesis, which is part of why a stressed or poorly lit coral calcifies slower even in perfect water. Calcium wants to sit between 400 and 550 ppm — right around what a natural reef runs — and once it drops below 360 ppm, growth on your corals, coralline algae, clams, and any Halimeda starts stalling out. Don't bother pushing it above natural levels chasing faster growth, though; once you're in range, extra calcium isn't buying you anything.

Alkalinity is really standing in for bicarbonate, which corals convert into carbonate to actually build that skeleton. Dissolved CO2, bicarbonate, and carbonate all exist together in one linked chemical equilibrium, and pH is really just a readout of where that equilibrium happens to sit at any given moment — alkalinity, pH, and dissolved CO2 aren't three separate things so much as three windows into the same underlying system. Corals actually spend metabolic energy pumping protons out of the calcification site to nudge that local equilibrium further toward carbonate, which is why a healthy, well-fed, well-lit coral can often calcify faster than the surrounding water chemistry alone would predict. Here's the twist: unlike calcium, corals often calcify faster with alkalinity running a bit above natural, because bicarbonate is the scarcer of the two to begin with. I'd keep it between 7 and 11 dKH — SPS-heavy, ultra-low-nutrient tanks specifically tend to do better sticking to the 7–8 dKH end, since a lot of people report "burnt tips" when alkalinity runs high or swings around. One thing worth knowing: the higher you run alkalinity, the more calcium and alkalinity your tank will burn through daily, and pushing it too high also drives calcium carbonate to precipitate out onto your heaters, pumps, and sand bed, wasting whatever you're dosing and hardening your substrate in the process.

Magnesium is the quiet third piece holding this whole system together. Both seawater and your tank are supersaturated with calcium carbonate, meaning it should be crashing out of solution constantly — and it would, if magnesium didn't step in. Magnesium ions are close enough in size to calcium that they get pulled onto the surface of a forming calcium carbonate crystal, but different enough that once they're wedged in, they distort the crystal lattice and raise the energy needed for the next layer to form, which stalls the process before it runs away. It's the reason the ocean has stayed supersaturated with calcium carbonate for millions of years without spontaneously turning to stone. Target 1250–1400 ppm. It does get consumed by growing coral and coralline skeletons, but usually only about a tenth as fast as calcium, so if it's dropping faster than that, suspect a bad test or a mismatched salt mix rather than actual demand.

For day-to-day maintenance of all three, a balanced system does the job — kalkwasser, a calcium reactor, or a two-part/three-part dose. If calcium has crashed hard and needs a fast one-time correction, skip the balanced stuff and dose calcium chloride directly instead, since balanced methods will overshoot your alkalinity trying to fix a calcium deficit. Same logic for alkalinity — baking soda for a gentle nudge, washing soda if you want the pH bump too. And don't correct magnesium by more than 100 ppm in a single day; spread bigger swings out over a few days.

The Environment Around Them

Salinity, temperature, and pH are less about feeding your corals and more about just not stressing them out.

Salinity should match the ocean — 35 ppt, specific gravity around 1.0264, conductivity around 53 mS/cm — unless you're keeping something specifically brackish or from the Red Sea. There's an old habit of running tanks a little low on salinity because people assume it's easier on fish; I've never seen anything that actually backs that up. It matters even more for your corals and invertebrates than for your fish, since most of them are what's called osmoconformers — their internal fluids simply track whatever salinity surrounds them, rather than being actively regulated the way a fish's blood chemistry is by its gills and kidneys. A swing in salinity hits a coral or a snail far more directly than it hits a fish, which has actual machinery built to correct for it.

Temperature is more of a trade-off than a target. Corals in the wild handle a huge range just fine, with the most diversity clustering around 83–86°F, but your tank isn't the ocean — it can't self-correct if the power goes out. Warmer water also holds less dissolved oxygen, which matters most during an emergency. Heat stress has a more specific danger too: it disrupts the photosynthetic machinery inside your coral's zooxanthellae, and those damaged algae start producing reactive oxygen species that harm both themselves and the coral tissue around them. The coral's defense is to expel the algae outright, which is what "bleaching" actually is — not the coral dying on the spot, but losing the partner it depends on for food and color, and it will slowly starve if the algae don't recolonize in time. I keep mine around 80–81°F, a touch cooler in summer and warmer in winter as a buffer against whichever direction a power failure would push it, and I'd stay somewhere in the 76–83°F range overall. Small daily swings aren't actually a problem either — tanks that see a little natural fluctuation tend to handle an unexpected spike better than ones locked dead-stable.

pH mostly takes care of itself once alkalinity is dialed in, and reef tanks do fine across a wider band than people assume — 7.8 to 8.55 is a reasonable acceptable range. As mentioned above, pH is just the visible face of that same CO2/bicarbonate/carbonate equilibrium running alkalinity, so the two are never really independent of each other. It's worth actually checking in two situations: if you're dosing kalkwasser, keep it under 8.55 or you'll drive excess precipitation onto your gear; and if you live in a newer, tightly sealed house, elevated indoor CO2 can chronically drag pH down, in which case more fresh air, a kalkwasser top-off, or a CO2 scrubber on your skimmer intake will fix it.

Feeding the Tank Without Feeding Algae

Phosphate, nitrate, and ammonia are all really the same story — nitrogen and phosphorus coming in through food, and what happens if you don't manage where it goes.

Phosphate mostly enters through feeding, and without some export method it just keeps climbing. The calcification problem isn't just correlation — dissolved phosphate binds onto the same growing calcium carbonate crystal surfaces that magnesium does, except instead of gently slowing the process the way magnesium does, it more aggressively jams new layers from forming at all, which is the literal mechanism behind "phosphate inhibits calcification." It also feeds algae — a lot of algae species are limited by phosphate below about 0.03 ppm, so keeping it low is one of your best algae-control levers. Go too low, though, and you risk pale corals and dinoflagellate problems. I'd target 0.06–0.3 ppm, managed through some mix of careful feeding, skimming, macroalgae or a turf scrubber, GFO, or carbon dosing.

Nitrate works the same way — target 5–50 ppm. It's not directly toxic at aquarium levels, but your coral's zooxanthellae take up nitrate as a nitrogen source to build proteins and reproduce, and too much of it lets the algae population inside the coral's own tissue grow faster than the coral itself can keep in check. That imbalance is exactly why an overfed, high-nitrate coral often looks darker and browner instead of more vibrant — you're growing more algae, not a healthier coral. Same export toolkit applies: less feeding, skimming, macroalgae, a deep sand bed, carbon dosing, or a denitrator. If it runs too low, food-grade sodium or calcium nitrate is a cheap fix, and dosed nitrate actually gives back a little alkalinity as it's consumed.

Ammonia is the one you genuinely don't need to test in an established tank — your bacteria and any macroalgae process it about as fast as it's produced. It matters during setup or right after adding new rock or sand, when there's suddenly more of it than your biological filtration can keep up with. Marine fish are tougher than the freshwater horror stories suggest, too; it typically takes well above 10 ppm to cause rapid harm. Ammonia and ammonium constantly interconvert depending on pH, and the uncharged ammonia form crosses cell membranes far more easily than the charged ammonium form does, which is the actual physical reason the exact same total-ammonia reading gets more dangerous as pH rises — relevant if you're ever bagging up fish for transport.

The Stuff You Can Mostly Ignore

This next group gets talked about constantly online, but the honest truth is most tanks run great without anyone ever testing for any of it:

Potassium stays in range on food and water changes alone for most people — only worth checking if you're running heavy carbon dosing and seeing poor growth or greyish Montipora. Silica is either a diatom problem (fix your source water) or, if diatoms aren't an issue, something you can gently dose with cheap sodium silicate to benefit sponges and mollusks. Iodine is chemically complicated and there's no solid evidence typical reef species need it supplemented — I dosed it for years, stopped, noticed nothing. Nitrite isn't worth testing once you're established; saltwater fish shrug off levels that would be lethal in freshwater. Strontium slowly depletes because it substitutes isomorphically for calcium in the growing crystal lattice — similar charge, similar ionic radius — which is exactly why it disappears right alongside calcium even though you never dosed it directly; it's toxic at high levels, so test before dosing rather than assuming more helps. ORP isn't something I'd chase a number on at all — it reflects the combined redox potential of every oxidizing and reducing compound dissolved in your water at once, which is exactly why a single reading is so hard to interpret and more useful watched as a trend than trusted as an absolute value. Boron gets marketed hard as a "buffer" but does very little of the actual buffering work, and salt mixes usually supply plenty. Iron matters mainly if you're growing macroalgae or running a turf scrubber, in which case a marine-formulated, chelated supplement dosed consistently is all you need.

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