What Are Soft Corals and How Do You Choose Them for a Saltwater Aquarium?

Soft corals are octocorals — corals whose polyps have eight tentacles and that build only tiny internal skeletal elements (sclerites) rather than the massive calcium carbonate skeleton of stony corals. That single structural difference explains most of what follows: soft corals are generally more forgiving of imperfect water chemistry, grow faster, and are the usual starting point for a new reef tank. They are a good choice if you want visible movement and rapid growth without committing to the calcium and alkalinity demand of a stony-coral system. They are a poor choice if you want a precisely sculpted, slow-growing reef where every coral stays exactly where you put it.

Soft corals vs. hard (stony) corals

The practical consequence: in a soft-coral tank you can get away with less precise testing and dosing. In a stony-coral tank you cannot.

Common groups and beginner-friendly examples

• Zoanthids and Palythoa — mat-forming polyps, many colour morphs, very tolerant. Handle with care and wear gloves; some carry palytoxin.
• Mushroom corals (Rhodactis, Discosoma, Ricordea) — single large polyps, low light tolerant, excellent first coral.
• Leather corals (Sarcophyton, Lobophytum, Sinularia) — the classic toadstool shape, fast growing, very hardy.
• Colt and finger corals — branching soft corals, good movement in flow.
• Xenia and Anthelia — pulsing polyps, dramatic but can spread aggressively and sometimes crash suddenly.
• Green star polyps (Pachyclavularia) — encrusting mat, extremely fast spreader.
• Kenya tree (Capnella) — drops branches that settle elsewhere; can become a nuisance.
• Gorgonians — photosynthetic and non-photosynthetic types; the non-photosynthetic ones need heavy feeding and are not beginner corals.

For a first purchase, mushrooms, toadstool leathers and zoanthids are the safest combination. Avoid non-photosynthetic gorgonians and, unless you accept the spread, Xenia and green star polyps.

Lighting, flow and water parameters

Soft corals as a group tolerate a wide range, but "tolerate" is not "thrive anywhere."

• Lighting: low to moderate. Many do well under basic LED fixtures. Too much light causes bleaching, shrinkage and colour loss in mushrooms and leathers.
• Flow: moderate, alternating flow is ideal. Enough to keep polyps moving and prevent detritus settling, but not a direct blast that keeps polyps closed.
• Temperature: stable tropical reef range, typically around 24–26 °C (75–79 °F).
• Salinity: stable reef salinity, typically around 1.024–1.026 specific gravity.
• Nitrate and phosphate: soft corals generally tolerate and even benefit from slightly elevated nutrients compared with stony corals. Very low nutrients can starve them.
• Calcium and alkalinity: less critical than for stony corals, but stability still matters. Large swings stress everything in the tank.

The key word across all of these is stability. A soft coral tank that holds steady at slightly imperfect values does better than one that swings between perfect ones.

Placement, spacing and aggression

Soft corals compete in two ways that matter for placement:

1. Chemical warfare (allelopathy). Leather corals in particular release compounds that suppress nearby corals, especially stony corals. Run activated carbon and leave space around large leathers.
2. Overgrowth. Fast spreaders like green star polyps, Xenia and Kenya trees simply grow over neighbours.

Practical placement rules:

• Put fast spreaders on an isolated rock island surrounded by sand, so they cannot cross to the main structure.
• Keep large leathers downwind (downstream) of stony corals, not upstream.
• Leave a gap of several centimetres between soft corals and anything you want to protect.
• Place mushrooms and zoanthids low, where light is gentler and they are easy to contain.
• Attach corals with reef-safe glue or by wedging the frag plug into rock; soft corals slime heavily, so glue works better on the plug or rubble than on the coral tissue itself.

Feeding, fragging and troubleshooting

Feeding. Most common soft corals are photosynthetic and get energy from their zooxanthellae, so they do not require target feeding. They do benefit from occasional fine particulate foods, and non-photosynthetic gorgonians require regular feeding of small planktonic foods. Overfeeding is a more common problem than underfeeding in a soft-coral tank because it raises nutrients.

Fragging. Soft corals are the easiest corals to propagate:

1. Remove the colony or frag from the tank.
2. For branching types (Kenya tree, colt, finger corals), cut a branch with sharp scissors or a scalpel.
3. For leathers, cut a wedge from the cap.
4. For mushrooms, cut the polyp across the mouth or slice off a piece of the foot.
5. Attach the cut piece to rubble or a plug with glue or by holding it in place with netting until it attaches.
6. Return it to moderate flow and light; expect a slime coat and a few days of sulking before it opens.

Common problems:

• Closed polyps for days: check flow, a recent change, or a nearby aggressive coral. Leathers also periodically shed a waxy layer and stay closed for several days — this is normal.
• Melting or disintegrating tissue: usually a sudden parameter swing, temperature spike, or bacterial issue. Improve stability, run carbon, and remove obviously dead tissue.
• Bleaching (loss of colour): too much light or too high temperature. Move to lower light and let it recover.
• Sudden collapse of Xenia: common and often unexplained; Xenia is a poor indicator of overall tank health despite its reputation.
• Overgrowth: the fix is manual removal and isolation, not a chemical treatment.

Choosing your first soft corals

Work through these in order:

1. Confirm your tank has stable temperature, salinity and nutrients, and that lighting is at least basic reef-capable LED.
2. Start with one or two mushrooms or a small toadstool leather.
3. Observe for two to three weeks — do polyps open, does colour hold?
4. Add zoanthids or a branching soft coral next.
5. Only add fast spreaders (Xenia, green star polyps, Kenya tree) once you have decided where they can be contained.
6. Buy tank-raised or aquacultured specimens where available; they are already adapted to aquarium conditions and are less likely to arrive stressed.

If your goal is a low-maintenance, movement-rich reef tank, soft corals are the right starting point. If your goal is a stony-coral-dominated system, treat soft corals as a separate zone or leave them out, because their chemical aggression and rapid growth work against the stability stony corals need.