Acropora humilis

Acropora humilis builds low colonies of thick, finger-like branches that taper toward the tip. At the tip of each branch is a large, dome-shaped axial corallite; the radial corallites are arranged in rows. It comes in cream, blue, brown and purple tones; the branch tips are often cream or blue. It is a species of wave-exposed reef crests and needs these conditions in the aquarium; small single-branch frags struggle to establish, so choose a piece with several branches. Trade in stony corals is subject to CITES Appendix II and the species is assessed as "near threatened" by the IUCN; buy only aquacultured or farmed colonies.

• Light: Very high light; roughly 300–500 PAR, acclimated over a period of weeks.
• Flow: Very strong, random flow. In weak flow the branches grow thinner and tissue loss begins.
• Placement: The very top of the rockwork; do not move it once placed, as every move can lead to a stall lasting months.
• Water parameters: Salinity 1.025–1.026, temperature 25–26.5°C, KH 7–9 dKH with daily swings below 0.5 dKH, calcium 400–450 mg/L, magnesium 1300–1400 mg/L, nitrate 2–10 mg/L, phosphate 0.02–0.08 mg/L.

It is photosynthetic; it gets most of its energy from zooxanthellae algae. Very fine plankton foods given occasionally can help, but light and water quality are what really matter.

It is not aggressive; keep it 10–15 cm away from LPS and soft corals. Dwarf angelfish may pick at it. Acropora-eating flatworms (AEFW), red bugs and rapid tissue necrosis (RTN) triggered by sudden KH changes are the main risks; put new pieces through a coral dip.