Coral Aggression & Spacing Checker

How the Coral Aggression & Spacing Checker Works

This coral aggression checker compares the selected main coral, the neighbor coral, the distance between them, whether they can touch when expanded, flow direction, growth form, relative position, and visible stress signs. The result is a risk level and a typical starting spacing, not a guarantee that two corals are compatible forever. Reef tanks vary by flow pattern, colony history, lighting, nutrients, water clarity, and the exact coral identity, so the safest answer is usually a range plus careful observation.

The calculator gives more weight to the factors that usually make a spacing decision urgent: close distance, direct contact, aggressive LPS sweepers, and current symptoms such as one-sided recession or sting marks. It also changes the result for slower problems such as overgrowth, chemical irritation, and shading. That means a Torch Coral near an Acan, Green Star Polyps growing toward Montipora, and a plating Montipora shading LPS should not return the same generic recommendation.

Why Coral Spacing Matters in a Reef Tank

Corals compete for space, light, and flow. Some corals sting directly when tissue touches. Some LPS corals extend sweeper tentacles that can reach beyond the visible daytime colony. Other corals grow over neighbors, release mucus or irritating compounds, or shade corals below as the colony gets larger. A small frag that looks harmless today can become a colony that changes the whole flow and light pattern around it.

Reef tank coral spacing is also directional. If flow pushes from an aggressive coral toward a peaceful neighbor, the same measured gap can be riskier than flow moving into open water. Position matters too: a leather coral or plating Montipora above another coral may become a shading issue even if there is no immediate sting.

Main Types of Coral Aggression

Sweeper tentacles

Sweeper tentacles are a common concern with aggressive LPS corals such as Torch Coral, Hammer Coral, Frogspawn, Galaxea, Pectinia, Favia/Favites, and many chalices. They are often easier to see after lights out. Flow direction matters because moving water can push tentacles toward a neighbor that appears safely out of reach during the day.

Direct contact and stinging

Can corals touch each other? Sometimes closely related corals are kept near each other, but direct contact is often risky unless you know the exact coral and have observed the colonies. Similar-looking corals are not always safe together. Torch Coral touching Hammer Coral or Frogspawn should be treated cautiously, especially when the ID is uncertain.

Overgrowth

Overgrowth is different from immediate stinging. Zoanthids, Green Star Polyps, Xenia, mushrooms, and some encrusting or plating corals can grow into neighboring bases or onto shared rockwork. Fast-spreading soft corals are often easier to manage on isolated rock where the mat can be trimmed or separated.

Chemical warfare and mucus

Some soft corals and mushrooms may irritate nearby corals through mucus, shedding, or chemical competition. Good flow, space, and carbon can help many mixed reefs, but they do not make crowded placement automatically safe. If a coral stays closed only on the side facing a neighbor, treat that pattern as a warning sign.

Shading

Large leathers and plating Montipora can shade nearby corals as they grow. SPS branches can also block flow and light around other colonies. Plan for the future colony size, not just the frag plug you are placing today.

General Coral Spacing Guidelines

Peaceful LPS often need several inches of space so fleshy tissue can expand without rubbing. Aggressive LPS often need 6 inches or more, with extra room for very aggressive species or flow that pushes tentacles toward neighbors. SPS corals need room for growth, shading, and strong flow around the colony. Fast-spreading soft corals are often safer on isolated rockwork. Unknown corals should get extra spacing until the ID is clearer.

These numbers are not universal guarantees. Torch coral spacing, hammer coral spacing, frogspawn coral spacing, chalice coral aggression, and favia coral aggression all depend on the actual colony, the neighbor, and the tank. Use the checker as a conservative starting point, then adjust based on what you see.

What to Watch for After Placing Corals

After moving or adding coral, watch for one coral staying closed, tissue recession on the side facing another coral, burn marks or pale patches, sweeper tentacles after lights out, corals touching when fully expanded, and fast mat growth reaching neighbors. If you see recession, sting marks, or sweepers contacting another coral, increase spacing rather than waiting for the damage to become severe.

What If You Don't Know the Coral ID?

Aggression risk depends heavily on coral ID. Torch, hammer, frogspawn, favia, chalice, galaxea, pectinia, acan, and Goniopora can be confusing for beginners, and a wrong ID can lead to a poor spacing decision. Use Coral Identifier as a fast starting point from a photo, then verify likely matches with trusted reef references before placing an unknown coral close to expensive or slow-growing neighbors.