As soon as a tank has real coral biomass in it, water changes alone often can’t replace calcium and alkalinity as fast as growing corals consume them — that’s the point where dosing enters the picture. There are several ways to do it, from simple two-part liquids to fully automated calcium reactors to a method built around dozens of individually tracked trace elements. None of them are as complicated as they look once you understand what’s actually being replaced and why.
What You’re Actually Dosing
Alkalinity (KH/dKH) is your water’s buffering capacity and the carbonate building block corals use, alongside calcium, to build their skeletons — it’s the single most important parameter to keep stable, since it moves fastest and swings hardest when something’s off. Calcium is the other half of that skeleton-building pair, the mineral corals pull directly from the water to calcify. Magnesium doesn’t get consumed nearly as fast, but it plays referee — it keeps calcium and alkalinity from precipitating out of solution together, and if magnesium drops too low (roughly under 1,200 ppm), your calcium and alkalinity dosing effectively stops working no matter how much you add. Trace elements are everything else in smaller supply — iodine, strontium, iron, manganese, and a long list of others that corals use in minor but real amounts for color, growth, and general health.
When to Start Dosing
A new tank with a handful of small frags doesn’t need dosing — routine water changes replace calcium and alkalinity faster than a lightly stocked tank consumes them. The signal to start is consumption outpacing what your water change schedule can keep up with: if alkalinity or calcium is measurably dropping between changes even though your water change volume and salt mix haven’t changed, your coral biomass has grown past what water changes alone can sustain, and it’s time to add a dosing method.
How to Figure Out How Much to Dose
Don’t guess — test your actual consumption first. Pause any existing dosing, test alkalinity, wait exactly 24 hours, and test again; the difference is your daily consumption rate (for more accuracy, repeat this for several days in a row and average it). A tank dropping from 8.3 to 7.9 dKH in 24 hours is consuming 0.4 dKH a day, and that’s the number you dose to replace — not a number you pulled from a label. Start any new dosing regimen at 50–70% of your calculated need for the first few days, then retest and adjust up gradually rather than dosing your full calculated amount immediately.
Two-Part Dosing
Two-part dosing is exactly what it sounds like: one solution for alkalinity, one for calcium, dosed in a matched ratio so neither runs ahead of the other. It’s simple, affordable, and effective, but most two-part solutions don’t include magnesium or trace elements, so those need separate supplementation as your tank matures — skip that and magnesium drift eventually causes both parts to stop working as intended. Dose the two parts at least 30 minutes apart in different high-flow areas, ideally split into multiple smaller doses throughout the day rather than one large dose, to avoid a local spike that causes precipitation.
Four-Part Dosing
Four-part dosing adds a dedicated magnesium bottle and a trace element bottle to the standard alkalinity and calcium pair, which is the more complete option for a mature, coral-heavy tank where all four are being consumed at a meaningful rate. It’s more bottles and more dosing schedule to manage than two-part, but it removes the guesswork of “when do I add magnesium separately” since it’s already built into the routine from the start.
All-in-One: Tropic Marin All-For-Reef
All-For-Reef is a single solution that covers calcium, alkalinity, magnesium, and trace elements together, which makes it about as low-effort as dosing gets — one bottle, one daily dose. Start around 5 mL per 100 liters (26 gallons) daily and increase by 2.5 mL per 100 liters weekly, using alkalinity as your guide, until you’re holding a stable 7–9 dKH. The tradeoff for the simplicity is less individual control over each parameter — if one element in particular is running low, you can’t adjust it independently the way you can with separate bottles.
Kalkwasser
Kalkwasser (calcium hydroxide, often just called “kalk”) is old-school but still effective — mixed into freshwater, it reacts with CO2 already in your tank water to release both calcium and alkalinity together, with a pH boost as a side benefit. The most common way to dose it is mixing it right into your auto top-off reservoir, so every drop of evaporated water that gets replaced also carries a small, steady dose of calcium and alkalinity. It has to be dosed slowly and into high flow, since dosing it too fast causes a dangerous pH spike and can cause the calcium and alkalinity to precipitate out instead of staying usable.
The Moonshiner’s Method
This is the deep end of dosing — instead of a handful of bottles, the Moonshiner’s method tracks and individually doses dozens of trace elements based on regular ICP testing, adjusting each one on its own rather than lumping them into a single blend. It’s the most precise approach available and popular with advanced SPS keepers chasing very specific coloration, but it’s also the most involved: it requires real testing infrastructure, careful record-keeping, and a level of attention most reef keepers don’t need or want. Worth knowing about, not something we’d recommend starting with.
Ideal Parameters by Tank Type
- SPS-dominant: Alkalinity 7–9 dKH, calcium 420–450 ppm, magnesium 1300–1400 ppm — SPS wants it stable and on the higher end of calcium.
- Mixed reef: Alkalinity 8–10 dKH, calcium 400–430 ppm, magnesium 1250–1350 ppm — a comfortable middle range that works across coral types.
- Soft coral / LPS-dominant: Alkalinity 8–12 dKH, calcium 380–420 ppm, magnesium 1250–1350 ppm — more tolerant of drift than SPS, with less need for tight precision.
Whatever range you land in, the number matters far less than holding it steady — the same principle that applies to nitrate and phosphate applies here too.
Why We Believe Consistent Bi-Weekly Water Changes Come First
Every dosing method above replaces a handful of specific elements — calcium, alkalinity, magnesium, maybe a defined trace element blend. A water change replaces everything at once, in the same balanced proportions found in real seawater, including the dozens of minor and trace elements no dosing regimen fully covers. That’s why we believe a consistent bi-weekly water change schedule is the right foundation for every tank, dosing or not: it’s the only maintenance step that resets your whole water chemistry back toward a known baseline rather than topping off a handful of numbers a test kit happens to measure. Dosing is a tool for keeping up with what a growing coral population consumes faster than water changes alone can replace — it’s an addition to a consistent water change routine, not a replacement for one.
The Bottom Line
Don’t dose until consumption testing shows you actually need to, and when you do, calculate your dose from real consumption data rather than a label estimate. Two-part is the simple, affordable starting point; four-part and All-For-Reef streamline magnesium and trace elements into the routine; kalkwasser is a proven low-cost classic; and the Moonshiner’s method is for advanced keepers chasing precision most tanks don’t need. Whatever you choose, keep bi-weekly water changes as the foundation underneath it. Questions about setting up a dosing routine for your tank? Reach out, or browse our current stock while you’re here.
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