Do corals really fight each other?

Yes. Corals compete for space and light using sweeper tentacles that reach out and sting neighbours, direct stinging where they touch, and chemical warfare (allelopathy) - releasing compounds that suppress other corals. In a small glass box these battles are up close, so placement and spacing matter.

Which corals are the most aggressive?

Euphyllia - hammer, torch and frogspawn - have long sweeper tentacles and should sit well clear of unrelated corals. Torch is the worst, out-stinging even other Euphyllia. Acans are semi-aggressive, and some LPS extend long sweepers at night. Give the aggressors room and check temperament before placing.

How far apart should I place corals?

Leave clear space between colonies, and give known aggressors like Euphyllia a wide buffer - a good rule is at least six inches from non-Euphyllia corals - plus room to grow into. Watch at night, when many sweeper tentacles extend much further than they do by day.

What is coral allelopathy?

It is chemical warfare: some corals, especially soft corals like leathers, release compounds into the water that suppress or stress other corals. You cannot see it, but it can stall a whole tank. Running activated carbon and good water changes helps keep these chemicals in check.

Why is my green star polyp or xenia taking over?

Because some soft corals spread invasively. Green star polyps, xenia and kenya tree can creep across rock and smother neighbours. Keep them isolated on their own rock or an island separated by open sand so they cannot spread onto your other corals.

Coral Aggression and Placement

🐠 The short version: corals compete — with sweeper tentacles, direct stinging, and invisible chemical warfare (allelopathy). The aggressors (Euphyllia, torch worst of all) need a wide buffer; the invaders (GSP, xenia, kenya tree) need isolating on their own rock; and soft-coral chemistry means you run carbon. Place corals by temperament as well as by light and flow, and leave room to grow.

🔧 Plan placement with the coral placement, light & flow guide — it carries each coral’s sourced spacing note alongside its light and flow zone.

How corals fight

A reef looks peaceful, but corals are locked in a slow war for space and light. Three weapons:

Sweeper tentacles — elongated tentacles that reach out (often at night) to sting anything nearby.
Direct stinging — where colonies touch, nematocysts (stinging cells) burn the loser’s tissue.
Allelopathy (chemical warfare) — compounds released into the water that suppress or stress other corals, tank-wide and invisible.

In an ocean these play out over meters; in your tank they play out over inches — which is why placement is a real skill, not decoration.

The aggressors — give them room

From our sourced coral care sheets:

Euphyllia (hammer, frogspawn, anchor) — long sweeper tentacles; keep them at least ~6 inches from non-Euphyllia corals. Hammers, frogspawn and anchors generally tolerate each other.
Torch — the exception and the worst. Torch out-stings even other Euphyllia, so give it its own space, away from hammers and frogspawn too.
Acans — semi-aggressive; they can extend sweepers at night. Space them from neighbours.
Watch at night, when many sweepers extend far beyond their daytime reach — that’s when you learn a coral’s true reach.

The invaders — isolate them

Some corals don’t sting so much as spread and smother:

Green star polyps (GSP), pulsing xenia, kenya tree — fast, weedy spreaders that creep across rock and overgrow neighbours. Keep them on their own rock or an island separated by open sand so they can’t march onto your other corals.
Toadstool and other leathers — the classic allelopathy offenders; they release compounds that stress nearby corals. Run activated carbon and keep up water changes to blunt the chemistry, and give them space.

The peaceful ones

Not everything is a warrior. Duncans and many soft corals are easygoing, and most SPS keep to themselves — though even peaceful corals need spacing to grow so they don’t eventually touch and start a border war. Peaceful doesn’t mean “no space required.”

A placement strategy

Put it all together:

Map by temperament — aggressors get buffers, invaders get isolated islands, peaceful corals fill the calm zones.
Layer in light and flow — each coral also has a light/flow zone (softies/LPS lower, SPS higher); the placement guide gives both the zone and the spacing.
Leave growth room — place for the colony’s future size, not today’s frag.
Run carbon — cheap insurance against allelopathy, especially in soft-coral-heavy tanks.
Observe at night — reposition anything whose sweepers are reaching a neighbour.

Get placement right and your corals grow into a settled, layered reef instead of a slow-motion turf war. It’s one of the skills that separates a mature reef from a struggling one.

Sources & further reading

Coralkin — Coral placement, light & flow guide — sourced spacing, light and flow per coral.
Coralkin — Coral care for beginners and the LPS / soft coral / SPS hubs.
Coralkin — Coral care questions.
Tidal Gardens & Bulk Reef Supply — coral aggression, sweepers and allelopathy (per our care sheets).