Coral compatibility and aggression: why corals sting each other

Why corals compete for space

Reef-building corals are colonial animals, permanently fixed to the substrate they settle on. On a natural reef, competition for space and light between neighbouring colonies is a major force shaping which species dominate a given area, studied extensively in coral reef ecology[1]. Researchers generally group coral competition into direct methods that require physical contact and indirect methods that don't[1]. It's worth keeping the individual mechanisms distinct, since they call for different fixes: overgrowth or shading, where a faster-growing or taller colony simply out-competes a neighbour for light and space without any specialised weapon; mesenterial-filament digestion, where a coral extrudes part of its own gut tissue onto an adjacent coral and digests it externally — a documented direct-contact attack distinct from stinging[1]; sweeper tentacles, a specialised stinging structure covered in its own section below; and allelopathy, chemical competition that needs no contact at all, also covered separately below.

In the wild, this plays out over reef structures spanning metres, and which mechanism dominates depends heavily on the specific taxa involved, their relative size, and local conditions — there's no single hierarchy where one mechanism always beats another. In a home aquarium, corals from entirely different reef environments are often placed within centimetres of each other, which is why aggression that would rarely matter at natural reef spacing becomes a routine practical concern for reef keepers.

Sweeper tentacles: direct stinging

Some coral species grow specialised "sweeper tentacles" — longer than the coral's normal feeding tentacles, with a distinct nematocyst profile from the coral's own ordinary feeding tentacles[2]. This tentacle-based competitive mechanism is documented in several genera, including Galaxea fascicularis, where a comparative histological and transcriptomic study found sweeper tentacles carry a different mix of stinging-cell types than the coral's catch tentacles, consistent with a specialised, aggression-adapted structure rather than simply a longer feeding tentacle[2]. In the reef-keeping hobby, sweeper tentacles are also widely reported in Euphyllia species — torch, hammer and frogspawn corals — which can extend well beyond the coral's visible base, often more actively at night, to sting anything they contact[3].

A sting from a sweeper tentacle doesn't need to be fatal to a neighbouring coral to cause real damage: repeated contact can cause localised tissue recession, bleaching at the contact point, or stalled growth on the affected side, even when the rest of the colony looks healthy.

Chemical warfare: allelopathy

Contact isn't required for corals to compete. Allelopathy — releasing chemical compounds that suppress a competitor without physical contact — is documented as a genuine, actively studied mechanism in coral competition, including in experiments showing algal turf and coral-derived chemicals measurably suppressing a competing coral's photosynthesis after contact or close proximity[5]. Soft corals in particular are widely reported by reef-keeping sources to release chemical compounds that can suppress the growth of nearby stony corals, even without any physical contact between them[3] — this is aquarium-observed and widely repeated hobbyist practical judgement rather than a specific controlled experiment on aquarium soft-coral species, distinct from the peer-reviewed turf/coral study just cited.

This is part of why mixing soft corals and stony (LPS/SPS) corals in the same enclosed water volume is a genuinely more complex stocking decision than temperament alone would suggest. Good water flow and protein skimming are commonly recommended in mixed-reef tanks and plausibly help by diluting and exporting dissolved compounds generally, but we haven't found controlled evidence that either measurably prevents or reverses allelopathic suppression specifically — treat them as a sensible general-husbandry precaution alongside spacing, not a guaranteed fix on their own. See our protein skimmer guide for the equipment side of that.

Practical spacing guidelines

Treat the figures below as a widely used hobbyist starting point, not a precise, universally agreed standard or a scientific rule — actual safe distance depends on the specific corals involved (not just which broad hobby category — SPS, LPS or soft coral — they're sold under), their colony size, their neighbour, your tank's flow pattern, and where they're placed relative to that flow. Two corals of the same genus can behave differently depending on colony size and how established each one is, and "LPS" or "soft coral" alone doesn't reliably predict how aggressive a specific specimen will be.

Whatever spacing you start with, plan for growth: a colony that looks appropriately spaced today can be in contact with a neighbour within months, particularly fast-growing soft corals and stony corals in favourable conditions. See our reef lighting guide for how lighting affects growth rate and placement decisions more broadly.

A genuine safety risk: Zoanthids and palytoxin

Reported effects range from tingling, nausea and muscle cramps to, in severe documented cases, respiratory depression requiring hospitalisation[8]. Public health guidance specifically recommends wearing gloves and eye protection when handling or fragging any zoanthid-type coral, never scrubbing, boiling or pressure-washing them, and working in a well-ventilated area rather than assuming normal aquarium maintenance precautions are sufficient[6]. This is directly relevant to a compatibility and spacing guide because the moment most exposures happen isn't routine viewing — it's exactly the kind of rearranging, fragging or removal this guide is otherwise about. If you or anyone in your household experiences unexplained symptoms after handling coral, this is a medical emergency requiring urgent medical advice, not something to wait out at home.

Signs of coral conflict vs illness

None of the signs below confirms a specific cause on its own, and more than one can look similar at a glance.

Fixing an existing conflict

Resolving coral aggression

1. 1

   Identify which coral is the likely aggressor

   Check which neighbouring coral has the longer reach (sweeper tentacles) or is a genus known for chemical competition, rather than assuming the coral showing damage is automatically the weaker or unhealthier one.

2. 2

   Increase physical distance, accounting for full reach

   Move one or both colonies so neither can contact the other even with tentacles fully extended, using the spacing guidance above as a starting point.

3. 3

   Improve flow and export if allelopathy is suspected

   Stronger water movement and functioning protein skimming are commonly recommended to help reduce dissolved chemical compounds in the water, on top of physical separation — a sensible general precaution, though we haven't found controlled evidence that either reliably reverses allelopathic damage once it's occurred.

4. 4

   Give the affected coral time before judging the outcome

   Coral tissue recovery is generally slow. Once the source of contact or chemical exposure is removed, look for stabilisation and new growth over weeks, not days, before deciding whether further action is needed.

Common mistakes

• Placing corals by visual composition alone, without checking which genera are known for sweeper tentacles or chemical competition.
• Measuring spacing from a coral's current visible size rather than its mature size and full tentacle reach.
• Assuming one-sided tissue damage is disease before checking what's actually within reach on that side.
• Handling, cutting or scrubbing Zoanthids without gloves, particularly with any cuts or broken skin on the hands.
• Mixing soft corals and stony corals in a small, low-flow volume without accounting for chemical competition, not just physical spacing.

Frequently asked questions

How AquaPulse can help

Coral conflict damage develops slowly, and by the time it's obvious, it's often hard to remember exactly when a colony was last moved or added relative to when the damage started. AquaPulse doesn't identify coral aggression or diagnose tissue damage — what it does is give your water testing and any notes you keep a shared timeline, so when you're trying to work out whether damage lines up with a recent rearrangement, you're checking a record instead of relying on memory. Get it on the App Store or Google Play.

Sources and uncertainty

Good to know: This guide provides general informational guidance only. The palytoxin safety information is not a substitute for professional medical advice — seek immediate medical attention for any suspected exposure symptoms.

• [1] Royal Society Open Science — "The war of corals: patterns, drivers and implications of changing coral competitive performances across reef environments" Peer-reviewed review. Used for the direct/indirect competitive-strategy framework, including overgrowth/shading and mesenterial-filament digestion as distinct direct mechanisms.
• [2] BMC Genomics (via PubMed Central) — "A tentacle for every occasion: comparing the hunting tentacles and sweeper tentacles, used for territorial competition, in the coral Galaxea fascicularis" Peer-reviewed study. Used for the specific finding that sweeper tentacles carry a different nematocyst and gene-expression profile from ordinary catch tentacles in Galaxea fascicularis, distinguishing a specialised competitive structure from an elongated feeding tentacle.
• [3] Reef Builders — "How to deal with Coral Aggression" Specialist reef-keeping hobbyist source. Used for Euphyllia sweeper-tentacle behaviour, soft-coral chemical competition and practical spacing figures. Not a scientific authority; presented as widely reported hobby consensus, not a controlled study.
• [4] Tidal Gardens — "Coral Aggression & Placement" Specialist reef-keeping hobbyist source, independent of source 3. Used as a second hobbyist reference corroborating the Euphyllia spacing figure. Not a scientific authority; presented as widely reported hobby consensus.
• [5] Coral Reefs (Springer Nature) — "Coral–algal competition: allelopathy, temporal variance, and effects on coral microbiomes" Peer-reviewed study. Used for the measured allelopathic suppression of coral photosynthesis on contact/proximity with competing organisms (algal turf and coral-derived chemicals specifically, not aquarium soft corals directly).
• [6] SA Health (Government of South Australia) — "Palytoxin Poisoning: Marine Aquarium Safety" Government public-health source. Used for handling precautions (gloves, ventilation, avoiding scrubbing/boiling), exposure routes, and confirmation that Palythoa (not Zoanthids generally) is the primary genus of concern.
• [7] Toxicon (ScienceDirect) — "Distribution of palytoxin in coral reef organisms living in close proximity to an aggregation of Palythoa tuberculosa" Peer-reviewed study. Used for the documented wide variation in measured palytoxin concentration between colonies, including samples with none detected.
• [8] StatPearls (NCBI Bookshelf) — "Coral Toxicity" Peer-reviewed medical reference. Used for palytoxin's mechanism, symptom range, onset timing and severity of documented human poisoning cases.

What we deliberately left out: a single universally agreed minimum spacing distance, since sources vary and actual safe distance depends on the specific corals, tank flow and mature colony size, and hobby-category labels like SPS/LPS/soft coral don't reliably predict it; a claim that all Zoanthid or Palythoa specimens contain dangerous palytoxin levels, since measured toxin content varies enormously by colony and isn't something a hobbyist can verify by appearance; a guarantee that flow, skimming or spacing alone reliably prevents allelopathic damage, since we found supporting husbandry practice rather than controlled evidence that any single measure reverses it; and a precise threshold tentacle length for "sweeper" classification, since this varies by species and individual colony.

Related guidance