Understanding reef tank water chemistry basics is one of the most important steps toward keeping healthy corals and fish. Stable parameters help prevent algae blooms, coral recession, and stressed livestock. In this guide, we’ll break down the key water parameters, what they do, and how to keep them in a safe range for a thriving reef.

Core Reef Tank Parameters and Ideal Ranges

While there are many things you can test, a stable reef usually focuses on these core parameters:

• Salinity: 1.025–1.026 specific gravity (35 ppt)
• Temperature: 77–79°F (25–26°C)
• Alkalinity (dKH): 8–9.5 dKH
• Calcium (Ca): 400–450 ppm
• Magnesium (Mg): 1250–1350 ppm
• Nitrate (NO₃): 2–15 ppm for most mixed reefs
• Phosphate (PO₄): 0.03–0.1 ppm

Each of these plays a different role. For example, alkalinity, calcium, and magnesium work together to support coral skeleton growth. Nitrate and phosphate are nutrients: too low and corals can starve; too high and algae can take over. For a deeper dive on nutrient control, check out our guide on controlling nutrients in reef tanks.

How the Big Three Work: Alkalinity, Calcium, and Magnesium

The “big three” parameters are the backbone of reef tank water chemistry. Keeping them in balance is more important than chasing a specific number.

Alkalinity: Your Stability Anchor

Alkalinity measures the water’s ability to resist pH swings. Rapid changes in alkalinity are one of the fastest ways to stress corals. Aim for consistency:

• Pick a target (for example, 8.5 dKH) and stay within ±0.3 dKH.
• Test at the same time of day for more consistent readings.
• Use a reliable test kit or digital checker.

Calcium and Magnesium: Coral Building Blocks

Calcium is used by stony corals, coralline algae, and clams to build hard skeletons. Magnesium helps keep calcium and alkalinity from precipitating out of solution.

• If calcium keeps dropping, your corals may be growing well or your salt mix may be low in Ca.
• If you struggle to maintain alkalinity and calcium, check magnesium first; low Mg often causes instability.
• Adjust slowly—no more than 50 ppm Ca or 100 ppm Mg per day.

Tip: Before adding any dosing system, track daily consumption of alkalinity and calcium for a week. This gives you a realistic baseline for your tank’s needs.

Once you understand your tank’s daily demand, you can move to manual dosing or an automatic doser. Our article on creating a reef tank maintenance schedule walks through how to fit testing and dosing into your weekly routine.

Keeping Nutrients in the Sweet Spot

Nitrate and phosphate often cause the most anxiety, but they don’t need to be zero. Corals benefit from a small amount of nutrients in the water.

• If nutrients are high: Increase export with larger water changes, better skimming, refugiums, or media like GFO for phosphate.
• If nutrients are too low: Feed a bit more, reduce aggressive filtration, or dose nitrate/phosphate supplements if needed.
• Change slowly: Rapid drops in nitrate or phosphate can shock corals just as much as high levels.

Regular testing, even once a week, helps you see trends before problems appear. For new tanks, we also recommend reviewing our post on cycling a saltwater aquarium so you understand how biological filtration ties into nutrients.

In the end, reef tank water chemistry basics come down to consistency. Choose reasonable target ranges, test regularly, and make small adjustments instead of big swings. With time and patience, your reef will reward you with growth, color, and long-term stability.

Sources

• Holmes-Farley, R. (Reefkeeping Magazine). “Reef Aquarium Water Parameters” and related chemistry articles.
• Sprung, J. & Delbeek, J. C. The Reef Aquarium series.
• Red Sea, ATI, and Tropic Marin salt mix parameter documentation.