Basic Chemistry: The Water Your Reef Lives In

Water chemistry is where most beginners feel lost, because it sounds like a college exam. It isn’t. For a reef, you only need to genuinely understand about five numbers, and one principle that matters more than all of them combined. This lesson gives you both.

Why this matters for your tank

Your corals are built out of the water around them. Their skeletons are literally minerals pulled from your tank. If those minerals run low, growth stalls and tissue suffers. If they swing up and down, corals get stressed even if the average is fine. Understanding these parameters is what turns water-testing from an anxious guessing game into a simple maintenance routine.

Core concepts

Salinity: how salty

Reef water should sit around 1.025 specific gravity (about 35 ppt). Too low stresses corals and inverts; too high does too. The main way salinity changes is evaporation: water leaves, salt stays, so the tank gets saltier over time. That’s why you top off evaporated water with fresh (RO/DI) water, not saltwater. Measure salinity with a refractometer for accuracy.

Alkalinity, calcium and magnesium: the building trio

These three work as a team to build coral skeleton:

Parameter	Reef target	Role
Alkalinity	8–9 dKH	Carbonate source + pH buffer
Calcium	400–450 ppm	Main skeleton mineral
Magnesium	1300–1400 ppm	Keeps the other two in solution

Here’s the key relationship: as corals grow, they consume alkalinity and calcium together. In a lightly-stocked new tank, weekly water changes replace what’s used. As coral demand grows, you’ll eventually “dose” these back in, but that’s a Lesson 10 concern. For now, just know they get used up, which is why they’re the parameters you’ll test most.

Magnesium is the quiet enabler. Without enough of it, calcium and alkalinity can’t stay dissolved at reef levels: they precipitate out. Keep magnesium in range and the other two behave.

pH: important, but don’t obsess

pH in a reef naturally rides up and down a little over each day (it dips at night when the tank consumes oxygen and produces CO₂, and rises during the day). A range of roughly 7.9–8.4 is healthy. Chasing a perfectly flat pH is a beginner trap; if alkalinity is stable and in range, pH mostly takes care of itself.

The principle that beats every number: stability

This is the most important sentence in the whole course:

Corals adapt to a range of conditions. What they cannot tolerate is rapid change.

A tank steadily running 7.7 dKH is healthier than one bouncing between 8 and 11 because someone keeps “correcting” it. Pick sensible targets, and then defend consistency. Slow and steady always wins.

In your tank

• Get a refractometer and a basic test kit (alkalinity, calcium, magnesium, plus ammonia/nitrite/nitrate for cycling, that’s Lesson 3).
• Top off evaporation with fresh RO/DI water to hold salinity steady.
• Test on a schedule, write it down. Trends over time tell you far more than any single reading. A cheap notebook or a notes app is fine.
• Change one thing at a time, slowly. If a number is off, correct it gradually over days, not in one big swing.

You now know what’s dissolved in the water and why it matters. But water chemistry only stays stable if a biological engine is running underneath it. That engine is the nitrogen cycle, and it’s the very first thing you have to build. That’s next.