The reach you cannot see at noon
Coral spacing mistakes often happen because daytime appearance hides part of the risk. Some corals extend sweeper tentacles well beyond their usual footprint, often after dark, and can damage neighbors they reach. A damaged patch is not proof of a sting, but it is a reason to check for contact as well as other causes. So allow room for full expansion, sweeper reach and growth, not just the frag you brought home.
The reach table
Measure the gap between expanded living tissues, not between frag plugs or colony centers. These examples describe risks, not guaranteed safe distances. Start generously, allow an extra buffer for growth and changing reach, and reassess. Common names and growth form do not reliably establish species or compatibility.
| Coral | Spacing approach | Why |
|---|---|---|
| Torch coral | Keep clear of neighboring corals, including hammers and frogspawn; allow for full extension and future growth | Head count does not establish a safe gap. Torches can sting hammers and frogspawn. |
| Galaxea | Allow a large buffer beyond its expanded tissue and observed sweeper reach | Galaxea fascicularis sweepers have been reported to reach about 30 cm (12 inches); this is not a guaranteed clearance or universal maximum. |
| Chalice and “favia”-type corals | Identify the coral where possible and allow generous separation; watch both sweepers and tissue contact | These trade groupings include different genera, with variable reach, growth and aggression. |
| Hammers and frogspawn | Keep neighboring corals outside full tissue and sweeper reach, with room to grow | Hammer sweepers are reported to reach about 6 inches. Some hammers and frogspawn tolerate close placement, but do not assume compatibility. |
Growth is the second spacing bill
Sting reach is only half the invoice. The other half is growth. Montipora growth varies by species, colony and conditions; plating or encrusting forms can overgrow or shade neighbors. Staghorn acros grow up and out and shade what sits under them. Encrusters spread across available surfaces. When you place a frag, plan for a larger colony and reassess as it grows, rather than assuming a fixed twelve-month size.
Rules that keep the peace
- Keep torches separate. Hammers and frogspawn often tolerate close placement with one another, but this is not guaranteed. Keep torches and these corals beyond each other’s reach and watch all neighbors for damage.
- Fast growers on removable islands. Separate rocks can make spreading corals such as green star polyps, zoanthids and encrusting Montipora easier to manage. Keep gaps clear and check for new growth; an island is not permanent containment.
- Mushrooms and anemones move. Some mushrooms slowly move or detach, and anemones can relocate. Leave room for expansion and keep checking their position.
- Check after dark. Look at different times after lights out, especially after adding or moving corals or changing flow, and keep checking as colonies grow. One inspection may miss sweepers, which can develop over days to weeks. If tissue is damaged, check for contact and other causes rather than assuming a sting.
- Flow stretches the battlefield. Flow can carry tentacles toward neighbors. Leave extra room in their path and reassess after pump or flow changes.
Before moving or trimming corals: Identify what you are handling. Zoanthids and palythoas may contain palytoxin. Never boil, heat, pressure-wash or scrub potentially contaminated rock. Seek experienced advice before removal or fragging, and follow palytoxin precautions, including suitable gloves, eye protection and ventilation.
Coral spacing in motion
A 35-second silent animation with on-screen text. Schematics are not to scale and do not predict growth speed or a safe gap. Open video.
Read the video transcript and visual description
0–7 seconds — Leave space for the colony.
A small frag can become a large neighbor. A branching coral gradually expands within an illustrative outline. Plan for mature size and full tissue extension. Growth shown is illustrative, not a timeline.
7–14 seconds — Match the gap to the form.
Branching, plating and encrusting corals use space differently. The examples grow upward and outward, form an overhang, or spread across rock. Allow for height and branch spread, overhang and shade, and rock-covering growth.
14–21 seconds — Measure from living tissue.
Allow room beyond the skeleton or frag plug. A generic fleshy coral extends moving sweeper tentacles. Sweeper reach varies by coral and conditions. No fixed buffer fits every coral.
21–28 seconds — Recheck after dark.
Observe again after moving corals or changing flow. Animated flow arrows and moving sweeper tentacles illustrate how flow can carry tentacles toward neighboring corals. Watch actual reach and recheck over time. A single observation may miss maximum extension.
28–35 seconds — Observe. Leave room. Adjust.
Plan beyond the frag. Allow for full tissue and sweeper reach. Observe after dark and after changes to flow. Leave room to adjust placement. No single distance guarantees compatibility.
All imagery is schematic and not to scale. Research the specific corals. The animation does not establish a safe distance or predict how fast a coral will grow. It contains no speech or music; all instruction is visible on screen.
The one inch rule is a myth
There is no universal safe distance, and any chart that gives one number for all corals is selling simplicity. Galaxea can have very long sweepers, while mushrooms still need room for expansion, movement and growth. Space by species, check it at night, and plant for the colony you are growing, not the frag you bought.
Sources and further reading: coral aggression and placement; sweeper-tentacle research; torch, hammer and frogspawn compatibility; Galaxea reach (p. 96); chalice and favia-type care; hammer sweeper reach; Montipora growth forms and variable growth rates; mushroom movement; anemone care.