SPS (small-polyp stony) coral is the most demanding category in the reef hobby. The skeleton-building corals - acropora, montipora, stylophora, pocillopora, seriatopora - form the foundation of natural reefs and the centerpiece of advanced aquariums. Their husbandry is binary: you're either growing them or losing them, with little middle ground.
SPS demand stable alkalinity, calcium, and magnesium because they consume those minerals at measurable rates while building skeletons. A reef with 10 acropora colonies will draw alkalinity down 1+ dKH per day - a tank without active dosing or a calcium reactor cannot keep up. Drip dosing or a calcium reactor with consistent output is mandatory once you have more than three or four colonies.
Light requirements are the second hard constraint. Standard SPS species (montipora, pocillopora, stylo) want 200-350 PAR. Demanding acropora (millepora, hyacinthus, tortuosa) want 300-450 PAR. Reef-spec LEDs (Radion XR15/30, AI Hydra, Reef Brite XHO supplemented with point-source) all work; T5 hybrid setups excel at deep penetration. Spectrum should be heavy in 420-460nm royal blue with violet supplementation.
Beginners should NOT start with acropora. Build a reef on softies and LPS for the first 12 months, develop the discipline to maintain alkalinity within 0.3 dKH and salinity within 0.001 SG, then add SPS. Skipping this step is why so many "first reefs" become brown sticks within 6 months.
Montipora capricornis (cap) and Montipora digitata. They tolerate parameter swings better than acropora, color up under modest light, and propagate easily.
Most likely an alkalinity swing (1+ dKH in 24 hours) or a light intensity change (sudden increase or decrease in PAR). Less commonly: phosphate spike, temperature swing, or pest infestation (AEFW, red bugs).
Drip-dosing with a peristaltic pump is the most stable approach. Two-part dosing once daily works for small systems. A calcium reactor is the long-term solution at 50+ gallons of SPS-dominant volume - it provides automatic, consistent output without daily intervention.