Zoanthids / Palythoa

Zoanthids and Palythoa are colonial polyps commonly kept in reef aquariums for their wide range of colors, patterns, and rapid growth. Although often grouped with soft corals in reef aquariums, zoanthids and palythoas are neither true soft corals nor stony corals. They belong to the order Zoantharia.

These colonial polyps grow by forming connected mats of tissue that spread across rockwork and aquarium surfaces. Individual polyps contain a central oral disc surrounded by tentacles and may vary greatly in size depending on the species. In general, zoanthids tend to have smaller polyps and thinner connecting mats, while many Palythoa species develop larger polyps and thicker tissue structures that may incorporate sand or sediment.

Zoanthids and palythoas are highly popular in reef aquariums because of their adaptability, coloration, and relatively fast growth under stable conditions. Many varieties are propagated extensively in captivity, resulting in a large number of selectively named color morphs within the hobby.

Most zoanthids and palythoas thrive in stable reef conditions with temperatures between 76–79°F, salinity around 1.025–1.026 specific gravity, and a pH between 8.0–8.4. Alkalinity is commonly maintained between 7.5–9 dKH, calcium around 380–450 ppm, and magnesium between 1250–1400 ppm. While these corals do not build large calcium carbonate skeletons, stable water chemistry still supports healthy growth and long-term health.

Zoanthids and palythoas generally tolerate a wider nutrient range than many SPS corals. Nitrate levels around 5–20 ppm and phosphate levels between 0.03–0.15 ppm are commonly acceptable for many species. Extremely nutrient-poor systems may lead to reduced growth, dull coloration, or poor polyp extension.

Most zoanthids and palythoas prefer low-to-moderate or moderate indirect flow. Proper flow helps keep debris and algae from accumulating between polyps while delivering oxygen and nutrients throughout the colony. Healthy colonies typically display full polyp extension, steady growth, and consistent coloration under appropriate conditions.

Excessive direct flow may cause polyps to remain tightly closed or stretched, while insufficient flow can allow detritus and nuisance algae to collect within the colony. Lighting requirements can vary considerably between species and morphs, with many adapting well to moderate lighting while others tolerate higher intensity placement.

Some species of Palythoa may contain palytoxin, one of the most potent naturally occurring toxins known. While serious exposure is uncommon, hobbyists should always use caution when handling, fragging, or cleaning zoanthids and palythoas. Wearing gloves, eye protection, and avoiding aerosol exposure during cutting or scrubbing is widely recommended for safe aquarium maintenance.

Although zoanthids and palythoas are often considered beginner-friendly, some species can spread aggressively and overgrow nearby corals if not managed properly. Careful placement and routine maintenance can help control growth within mixed reef aquariums.