Brief Look Into Alkalinity

If I had to pick one number to obsess over in my reef tank, it’d be alkalinity. Not calcium, not salinity. Alkalinity. When I started, I fixated on calcium because it sounds more important (corals build calcium carbonate skeletons, after all), but alkalinity is the parameter that actually holds everything together.

Think of alkalinity as your water’s shock absorber. Every biological process in the tank pushes pH around. Fish breathing, waste breaking down, coral growth. All of it. Alkalinity is what keeps those hits from turning into wild swings. It’s a buffer. When the buffer runs out, everything downstream gets wobbly.

Alkalinity (sometimes labeled KH or dKH on test kits) is made up of bicarbonate and carbonate ions. They do two things: stabilize pH and give corals the raw material to grow skeleton. Two jobs, one parameter.

The big three

Alkalinity doesn’t exist in a vacuum. It’s locked in a relationship with calcium and magnesium, and you can’t manage one without understanding the other two.

Alkalinity and calcium are joined at the hip. Corals consume both when they build skeleton. Dose one without the other and things fall out of balance fast.

Magnesium is the reason calcium and carbonate don’t just crash out of solution on their own. It keeps them from spontaneously reacting, which is why it’s the third number everyone tracks. You’ll hear this trio called “the big three.” Once I started tracking all three together instead of chasing one at a time, things clicked.

The number to aim for

Most reefers target somewhere between 8 and 12 dKH. Natural seawater sits around 7–8. I keep mine around 8.5. Stable, but the corals aren’t starving.

Some people run higher to push faster coral growth, but that means more dosing and more testing. The tradeoff usually isn’t worth it unless you’re ready to babysit.

The actual number matters less than keeping it steady. Swings of more than 1 dKH in a single day can stress corals. Ideally, keep daily variation under 0.5. They don’t care if you’re at 8 or 10. They care if you’re at 8 in the morning and 9.5 by dinner.

When it breaks

Too low

Let alkalinity crash and your corals will let you know about it. Growth slows down. Sometimes it stops entirely. Your pH starts swinging because there’s nothing left to buffer it. Stuff that normally doesn’t matter suddenly does.

Push it far enough and you get tissue loss. Bleaching. The bad stuff.

Too high

You get alkalinity burn on coral tips. White, receding edges that look like the coral is melting from the top down. Calcium starts dropping because the two parameters are always fighting for balance. And if things get really out of hand, calcium carbonate precipitates out of solution on its own. You’ll see white chalky buildup on heaters, pumps, anywhere there’s flow.

Keeping it stable

How fast your alkalinity drops depends mostly on what’s in the tank. A tank packed with SPS and LPS stony corals will chew through it way faster than a soft coral setup or a fish-only system. When I first started, water changes were enough. As the corals grew in, I had to start dosing.

There are a few ways to do it.

Two-part dosing is where I started, and where most people start. One bottle for alkalinity (soda ash or sodium bicarbonate), one for calcium (calcium chloride). Simple, balanced, hard to mess up.

Kalkwasser bumps alkalinity, calcium, and pH all at once. Sounds great until you dose too much and your pH spikes. It also can’t keep up with heavy demand on its own. Good for tanks that run low on pH, though.

If you want to stop thinking about it entirely, calcium reactors dissolve media to release both calcium and alkalinity. More upfront setup, less daily fiddling.

Whatever you pick, test at the same time every day. You’re tracking how fast your tank eats through alkalinity so you can dial in the dosing. I split my daily dose into small hits every hour. Keeps things flat. The corals don’t notice anything changing, which is the whole point.

The short version

Alkalinity is your tank’s shock absorber. Track it, keep it steady, and don’t chase a number. Just keep it flat.

Sources

1. Manta Systems, alkalinity overview
2. ATI North America, alkalinity basics (2021)
3. Bulk Reef Supply, alkalinity basics (2021)
4. Tidal Gardens, alkalinity
5. Fauna Marin, carbonate hardness knowledge base
6. Reef Builders, calcium, alkalinity, and magnesium by Michael Paletta (2023)
7. Hanna Instruments, testing alkalinity in saltwater (2024)