Zoanthid Care for Beginners: Light, Flow, Feeding and Safety

A three-polyp frag goes on a rock in the spring. By the next summer it has crawled across that rock and started on the one next to it. That is why zoanthids are most people’s first coral. The other reason to learn about them before you buy is that some of them carry one of the most potent toxins known.

This guide covers keeping zoas from the first frag to a full colony: handling them safely, light, flow, water, feeding, fragging, pests and names. If yours are closed up right now, start with why zoanthids won’t open instead.

What zoanthids are

Zoanthids are colonial animals related to anemones. Each polyp is a small disc ringed with tentacles on a short stalk, and the polyps are joined by a mat of tissue that spreads across rock. They have no stony skeleton, so they don’t take up calcium or alkalinity the way stony corals do. Bulk Reef Supply files them with the soft corals.

“Zoas” usually means the genus Zoanthus. “Palys” means Palythoa and Protopalythoa. You can usually tell them apart:

	Zoas (Zoanthus)	Palys (Palythoa)
Polyps	Smaller, set in a thin mat	Larger discs, shorter tentacles
Tissue	Thin, clean	Thick, with sand and shell bits worked in
Color	Bright, busy patterns	More muted
Feeding	Rarely seen eating	Eats readily

They are tough because of where they come from. Eric Borneman described them growing on wave-pounded shorelines and in tide pools that dry out at low tide, and wrote that they “will tolerate wide fluctuations in water parameters in the captive environment and still flourish.” They grow wild here in the Keys too. The white encrusting zoanthid, Palythoa caribaeorum, covers reef in the Florida Keys National Marine Sanctuary.

Safety first: palytoxin

Some zoanthids contain palytoxin, which an FDA-led study in 2011 called “one of the deadliest toxins ever discovered.” In that study, researchers tested 16 zoanthids from aquarium stores and one home tank. Four were highly toxic, and all four were the same type of brown Palythoa. Every Zoanthus they tested was non-toxic or weakly toxic.

That is reassuring, but it doesn’t make any zoa safe to handle carelessly. You can’t tell a toxic colony from a safe one by looking at it, trade names say nothing about species, and nothing sold in the U.S. is tested or labeled. The CDC documented 10 cases in Alaska, including a home where three people got sick without ever touching the coral. Other published cases include people poisoned by the steam from boiling rock, a family sickened after one person cleaned coral in a basement sink with hot tap water, and eye injuries bad enough to need corneal transplants.

Light: 75 to 150 PAR

Most farms land in the same range. Tidal Gardens keeps its zoas at 75 to 125 PAR, Top Shelf Aquatics recommends about 80 to 150, and Bulk Reef Supply says 75 to 250. Start around the middle of that range.

A new frag goes lower in the tank or in a shaded spot first. Tidal Gardens puts it this way: a new coral is “far more likely to be damaged from overexposure than starve from underexposure.” Move it up over days to weeks, or use your light’s acclimation mode, and watch the polyps. Stalks that stretch toward the light mean it wants more. Polyps that close up or lose color mean it’s getting too much.

Flow: enough to keep them clean

Zoas want steady, indirect flow, strong enough to keep detritus from settling on the mat but not so strong it blasts the polyps shut. Tidal Gardens says flow is the one thing it “would obsess over” with zoas. Flow also changes their shape. Borneman found that in strong current they grow short, stiff polyps with small discs in low, dense mats, and in low flow the polyps grow larger with longer tentacles.

Keep zoas on rock, not the sand bed. Tidal Gardens notes that zoas “do not like substrate,” though palys handle it fine.

Water: stable, with some nutrients

Parameter	Target
Temperature	75 to 78°F
Salinity	1.025 to 1.026
pH	8.1 to 8.4
Alkalinity	8 to 9.5 dKH
Calcium	400 to 450 ppm
Magnesium	1300 to 1450 ppm
Nitrate	10 to 20 ppm
Phosphate	0.05 to 0.1 ppm

The nutrient numbers matter more than they look. Tidal Gardens aims for 10 to 20 ppm nitrate and 0.05 to 0.1 ppm phosphate, and warns that near-zero nutrients shrink the polyps and can set off dinoflagellate blooms. In the wild, big zoanthid colonies grow near nutrient-rich outflows. Top Shelf Aquatics sums up the rest: “Stability is the biggest key.”

Feeding is optional

Zoas live mostly off light. In a 1971 study of Hawaiian zoanthids, Amada Reimer wrote that Zoanthus “has never been observed to feed” in nature, while Palythoa actively eats zooplankton. Borneman added that light alone never covers a zoanthid’s whole energy budget, and World Wide Corals says feeding isn’t required but speeds growth.

If you want faster growth, Tidal Gardens turns the pumps off, dusts the colony with fine frozen foods like mysis fines, cyclops and rotifers, waits about 10 minutes, and turns the pumps back on. Bulk Reef Supply suggests one to three times a week. Watch your fish. Some learn to steal from the polyps and then pick at the colony all day, which keeps it closed.

Growth and spacing

Borneman once watched six or seven Zoanthus polyps cover about a square foot of rock in a year and a half. Growth depends on the morph, light, flow and nutrients, so plan for weeks to months, not days. Two things slow it down: shipping stress, which Tidal Gardens says can stunt growth for up to a year, and single-polyp frags, which fish and hermit crabs pick off easily. Buy frags of at least three or four polyps.

Zoas spread sideways and will crawl onto neighboring corals. Bulk Reef Supply suggests giving them their own rock “islands” to keep them in check. They are also weak fighters, so keep them away from corals with long sweeper tentacles.

Fragging and gluing

Fragging means taking coral out of the water, so keep it short and wear gloves and eye protection. Work over a shallow dish of tank water. Two methods work:

• Cut a clump of 6 to 10 polyps off the mat with a new scalpel blade or coral scissors, following the natural breaks between polyps and cutting as little tissue as you can.
• Or cut a small piece of the rock the polyps sit on with bone cutters, and glue the whole piece to a plug.

Pat the base dry, put glue on the plug and the underside of the frag, and set it with tweezers, not fingers. Cut zoa tissue can squirt fluid when you press it, so keep your face back. Rest the new frags in lower light and mild flow. Within a few days the polyps should open and the frag should start to attach.

Pests and dipping

Zoa pox

Small white pimples on the stalks. Nobody has confirmed the cause. Tidal Gardens links it to wild colonies and to low flow that traps detritus. More flow and a clean colony are the usual response.

Zoa-eating nudibranchs

Brown, frilly and about half an inch long, often colored like the zoas they eat. The eggs are small spirals laid on the sides of the polyps. Signs are polyps that never fully open and a slow decline. Dip every 4 to 7 days until you stop finding them, and pick off egg spirals by hand, since dips don’t kill the eggs.

Sea spiders

Tiny, long-legged animals at the base of the polyps, easiest to spot after lights out. They suck out tissue and can wipe out a colony. Dip repeatedly and pull them off with tweezers.

Sundial snails

A flat, spiral shell with a black and white checkerboard pattern, usually tucked between the polyps. They eat only zoas. Remove them by hand as soon as you see one.

Large brown Asterina stars, some crabs, and fish such as tangs, foxfaces, angelfish and butterflyfish can also eat or pick at zoas. Most of these pests come in on wild colonies, which is one reason farm-grown zoas are cleaner.

Why the names don’t match

The same zoa can sell under three names, and one name can cover zoas that look different. Reef Builders explained it in 2014: named corals sell better, so vendors invent names. Top Shelf Aquatics describes Rastas as orange, yellow, green, blue and a hint of purple, while Tidal Gardens calls theirs red, yellow and green. Halle Berry is sold as a zoa by one vendor and as a paly by another.

Photos add to the confusion. Coral photos are often shot through an orange filter or color-corrected to cut the blue light, so a zoa can look different in your tank. The safest buy is the exact colony in the picture. That is why we sell so many of our corals as WYSIWYG.

Beginner mistakes

• Handling zoas bare-handed or without eye protection, or boiling and scrubbing rock that has zoas on it.
• Skipping the dip, and keeping the frag plug it came on without cleaning it.
• Putting a new frag straight into full light.
• Chasing zero nitrate and phosphate.
• Leaving zoas in a dead spot where detritus settles, or on the sand bed.
• Buying single polyps and keeping them with hermit crabs or grazing fish.
• Paying extra for a name instead of the colony you can see.

Quick answers

Are zoanthids toxic?

Some are. The proven high-risk ones in testing were a type of brown Palythoa, but no zoa sold in the U.S. is tested, so handle all of them with gloves and eye protection.

Are zoanthids soft corals or LPS?

Soft. They have no stony skeleton and don’t use calcium or alkalinity.

Do zoanthids need to be fed?

No. They live mostly off light. Occasional fine food speeds up growth.

Can zoanthids grow on sand?

Zoas do better on rock. Palys handle the sand bed.

Sources

• Deeds et al. (2011), palytoxin in aquarium zoanthids, PLOS ONE
• CDC, palytoxin exposures in Alaska (MMWR, 2015)
• Eric Borneman (1998), zoanthids in the aquarium
• Amada Reimer (1971), feeding behavior in Hawaiian zoanthids, Pacific Science
• Tidal Gardens, zoanthus care and troubleshooting
• Bulk Reef Supply, zoanthid care guide
• Reef Builders, the trouble with coral trade names (2014)

The Underwater Gardener grows aquacultured corals in Key West, Florida (FL Aquaculture Certificate #AQ0254072). Our corals are raised under stable reef lighting in our own systems, so they arrive already adapted to a well-run tank. Corals ship UPS Next Day Air every Tuesday and Wednesday.