SPS Dosing Guide: Keeping Up with Demand

Dosing: Because Water Changes Alone Will Not Cut It

SPS corals build skeletons out of calcium carbonate daily. A growing SPS tank can consume alkalinity and calcium faster than water changes can replenish. Once there are more than a handful of SPS frags, a dosing strategy becomes necessary.

What to Dose and Why

Alkalinity: The fastest to deplete. SPS consume it for skeleton building — a drop in alk is usually the first sign corals are outpacing supplementation
Calcium: Used alongside alkalinity. Generally depletes slower but still needs supplementing
Magnesium: Stabilises the alk/cal relationship. Depletes slowest and sometimes water changes alone keep up

Dosing Methods

Two-Part Dosing (Most Common)

Equal parts of an alkalinity solution and a calcium chloride solution. Popular UK brands include Red Sea Reef Foundation A+B, Tropic Marin, or DIY from bulk chemicals.

For the budget route: a 5kg bucket of sodium bicarbonate and a bag of calcium chloride costs around £20 and lasts months. Bake the sodium bicarbonate at 150°C for an hour to convert it to sodium carbonate, then dissolve 200g per litre of RODI water for part A. Part B is 133g calcium chloride dihydrate per litre of RODI.

All-in-One Products

Products like Tropic Marin All-For-Reef combine alk, cal, and mag in one solution. Convenient for smaller tanks, but ratios cannot be adjusted independently.

Kalkwasser (Limewater)

Calcium hydroxide mixed with top-off water. Supplements both calcium and alkalinity, helps maintain pH, and precipitates phosphate. Budget-friendly but less precise.

Calcium Reactor

A reactor dissolves aragonite media using CO2. High initial cost (£200–400+) but very low running cost and extremely stable output. Often where heavily stocked SPS tanks end up.

Dosing Pumps

Manual dosing works initially, but SPS prefer small frequent doses over one daily dump. Good UK options:

Jebao DP-4 — budget option, around £50–70
GHL Doser — mid-range, very reliable, £150–250
Kamoer FX-STP — wifi-connected, £80–120

Set the pump to dose in 4–6 small increments over 24 hours for minimal parameter swings.

Dialling In

Test daily for the first week or two
Calculate daily consumption from the test results
Start conservative and increase gradually
Retest weekly once stable — consumption increases as corals grow

What dosing method works best at different tank sizes? Share what is working below.