Keeping SPS corals thriving comes down to rock-solid stability, and nothing matters more than dialing in calcium and alkalinity. When these two parameters are in balance, SPS reward you with fast growth, rich color, and strong skeletons. This guide walks you through the targets, testing, and dosing strategy you need for a healthy SPS reef.

Ideal Calcium & Alkalinity Ranges for SPS

Stony corals build their skeletons from calcium carbonate, so both calcium (Ca) and alkalinity (dKH) must be kept in the right range and stable day to day.

Target Parameters

• Calcium: 400–450 ppm (aim for a specific number like 420 ppm and stick close to it)
• Alkalinity: 7.5–9 dKH (pick a target, for example 8.0–8.5 dKH, and keep it steady)
• Magnesium (supporting role): 1300–1400 ppm to help stabilize both Ca and Alk

More important than chasing “perfect” numbers is avoiding big swings. A jump of 1.5–2 dKH in a day can stress SPS, leading to burnt tips, tissue recession, or color loss.

For a deeper look at basic water chemistry before you fine-tune SPS parameters, check out our Reef Tank Water Chemistry Basics guide.

Testing & Interpreting Your Results

Consistent testing is the backbone of SPS success. Manual test kits are usually enough for smaller or lightly stocked systems, while automated testers shine on heavy SPS tanks.

Testing Routine

• Alkalinity: Test daily when dialing in dosing, then 2–3x per week once stable.
• Calcium: Test 2–3x per week at first, then weekly once your usage rate is known.
• Magnesium: Test every 2–4 weeks or after major changes.

Always test at the same time of day. Alkalinity can fluctuate slightly over 24 hours, so consistent timing gives you cleaner data. Log your results in a notebook or app so you can spot trends and adjust dosing before problems appear.

Tip: If calcium and alkalinity both keep drifting down between water changes, your corals are outgrowing what your salt mix alone can supply. It’s time to add a dedicated dosing method.

Dosing Strategies for Stable SPS Growth

Once you know your tank’s daily consumption, you can choose a dosing method that matches your system and maintenance style.

Common Methods

• Two-Part Dosing: Separate calcium and alkalinity solutions, usually dosed with pumps. Easy to fine-tune and ideal for most SPS tanks.
• Kalkwasser (limewater): Added via ATO or slow drip. Provides both calcium and alkalinity plus a pH boost, but limited by evaporation rate.
• Calcium Reactor: Dissolves media with CO₂ to supply Ca and Alk. Excellent for high-demand SPS systems but more complex to set up.

Practical Dosing Tips

• Increase or decrease doses in small steps (no more than 0.5 dKH per day change).
• Space calcium and alkalinity doses at least 15–30 minutes apart to reduce precipitation.
• Re-test 24 hours after any dosing adjustment to confirm the new trend.
• Pair your dosing plan with regular, consistent water changes to replenish trace elements.

For help choosing equipment that matches your system size and coral load, visit our SPS Reef Tank Setup Guide and our article on Automatic Dosing Pumps for Reef Tanks.

Bringing It All Together

Successful SPS reef keeping is less about chasing numbers and more about keeping them steady. Lock in a reasonable target for calcium and alkalinity, test consistently, and adjust your dosing in small, deliberate steps. With stable chemistry, your SPS will reward you with strong growth, vivid coloration, and long-term health.

Sources

• Holmes-Farley, R. (Reefkeeping Magazine). “Calcium and Alkalinity” series.
• Borneman, E. (2001). Aquarium Corals. TFH Publications.
• Sprung, J. (2005). Corals: A Quick Reference Guide. Ricordea Publishing.
• Manufacturer documentation from major reef salt and additive brands (Red Sea, Tropic Marin, Brightwell).