Calcium Reactors vs Two-Part Dosing: Which Is Right for Your UK Reef?

The Question Every SPS Keeper Eventually Asks

Once stony corals start growing properly — visible encrustation and branching — alkalinity and calcium start dropping fast. Corals consume both to build skeletons, and that consumption needs replacing. The two main approaches: two-part dosing and calcium reactors.

Two-Part Dosing: The Accessible Option

Two separate solutions — calcium chloride and sodium bicarbonate — dosed in balanced amounts. Most reefers automate this with dosing pumps (Jebao, Kamoer, or GHL).

Popular products: Red Sea Foundation A, B, C (£30–40), Triton Core7 (£45–55), or DIY two-part from bulk suppliers (~£15–20 for months of supply).

Pros: Low upfront cost (~£100–150), simple to adjust, fine-grained control, scales easily.

Cons: Adds sodium/chloride over time, requires regular testing, heavy consumers go through a lot of solution, and pump failures can cause parameter swings.

Calcium Reactors: The Set-and-Forget Option

A sealed chamber of aragonite media through which CO2-acidified tank water slowly recirculates, dissolving the media and releasing calcium, alkalinity, and magnesium in natural ratios.

What's needed: Reactor chamber (£150–350), CO2 bottle, regulator with solenoid (£40–80), pH controller (£60–100), and reactor media (~£15–25/bag).

Pros: Replenishes Ca/Alk/Mg in natural ratios, no sodium chloride buildup, minimal ongoing maintenance, cost-effective long-term.

Cons: High upfront cost (£400–600+), lowers tank pH (problematic in poorly ventilated UK homes), takes time to dial in, overkill for small tanks.

Which to Choose?

Two-part suits tanks under 300L, growing coral collections, and consumption under 2 dKH/day. Calcium reactors suit 300L+ tanks with significant SPS, consumption above 2–3 dKH/day, and stable pH above 8.0. Some reefers run both — a reactor for bulk supplementation with two-part fine-tuning.

Browse filtration equipment from UK sellers — calcium reactors hold their value well and good deals on used setups are common.

Which camp are you in — two-part or calcium reactor? Or something else entirely?