Keeping calcium and alkalinity stable is one of the biggest challenges in a thriving reef tank. A well-tuned calcium reactor can turn that daily dosing struggle into a smooth, automated process that keeps your corals growing and your parameters rock solid.

What Is a Calcium Reactor and How Does It Work?

A calcium reactor is a sealed chamber filled with aragonite media that slowly dissolves in low-pH water, releasing calcium, alkalinity, and trace elements back into your aquarium. Instead of manually dosing two-part every day, the reactor continuously supplies what your corals consume.

Key components of a typical calcium reactor setup include:

• Reactor chamber – Holds the media and recirculating pump.
• CO₂ system – Regulator, solenoid, and CO₂ tank to lower pH inside the reactor.
• Feed pump or manifold – Pushes tank water through the reactor.
• Effluent line – Returns calcium- and alkalinity-rich water to your sump.

Inside the reactor, CO₂ is injected to bring the pH down (typically 6.3–6.8). At this lower pH, the aragonite media dissolves, creating an effluent that’s rich in calcium and carbonate. That effluent drips back into your system, replacing what your corals and coralline algae use up.

Setting Up and Tuning a Calcium Reactor

Before adding a reactor, make sure you understand your system’s daily consumption. If you’re still learning how to track and stabilize parameters, start with our guide to maintaining stable alkalinity and then step up to a reactor once demand is consistent.

Basic Setup Steps

• Rinse and fill the reactor with quality aragonite media.
• Connect the feed line from your return manifold or a small peristaltic pump.
• Install the CO₂ regulator, bubble counter, and solenoid on your CO₂ cylinder.
• Route the effluent line to a high-flow area of the sump.
• Calibrate your pH probe if you’re using a controller.

Tuning for Stable Parameters

Think of tuning a calcium reactor as balancing two levers: CO₂ bubble rate and effluent flow.

• Start conservative: Aim for a slow drip (30–60 mL/min) and a pH of around 6.7–6.8 in the reactor.
• Test often: Check alkalinity daily for the first week. If it’s dropping, either increase the bubble rate slightly or increase effluent flow.
• Make one change at a time: Adjust in small steps and wait 24 hours before changing again.
• Use a controller if possible: A pH controller that turns CO₂ on and off within a range (for example, 6.6–6.8) adds a big safety margin.

For tanks with heavy SPS demand, a reactor can be paired with a kalkwasser system for additional pH support. If you’re curious about that approach, see our overview of kalkwasser for reef tanks.

Tips, Maintenance, and When a Reactor Makes Sense

A calcium reactor shines on medium to large systems with high coral growth. If you’re constantly increasing your two-part dosing, it may be time to consider a reactor.

Practical tips for long-term success:

• Monitor pH in the display: Reactors can nudge pH downward. Running the effluent into a refugium or skimmer section helps gas off excess CO₂.
• Service the pump: Clean the recirculation pump every 3–6 months to prevent flow issues.
• Top off media early: Refill when the chamber is about half empty to keep performance stable.
• Check CO₂ pressure: Low tank pressure causes erratic bubble rates—swap cylinders before they’re completely empty.

Compared to manual dosing, a calcium reactor has a higher upfront cost but offers unmatched long-term stability and convenience. Once dialed in, it quietly supports dense coral growth with minimal daily attention.

If you’re planning a high-demand reef and want to design it around stable chemistry from day one, don’t miss our full reef tank planning guide to map out equipment, stocking, and maintenance together.

With careful setup and patient tuning, a calcium reactor can become the backbone of your reef’s stability—keeping calcium and alkalinity steady so you can focus on enjoying healthy, fast-growing corals.