Zoanthids are supposed to be the easy corals. Which makes it especially maddening when a colony that has been spreading happily for a year suddenly closes up, develops white or grey crusty patches on the polyp skirts, and starts melting away one polyp at a time.
What zoa pox looks like
The classic presentation: small white or light-coloured pitted spots on the outside of the polyp, usually on the column and the skirt rather than the mouth. Polyps stay closed, or open weakly and briefly. It spreads across a colony over days to weeks, and it tends to hit one colony hard while a colony six inches away is untouched.
Two things worth being honest about. First, “zoa pox” is a hobby label, not a confirmed single pathogen — it is generally understood as a bacterial infection, and it may well be several different things that look alike. Second, that means treatment is empirical: reefers do what has worked, not what a lab confirmed.
What it is not
Before you treat anything, rule out the far more common causes of closed zoas:
- Zoanthid-eating nudibranchs — perfectly camouflaged, they match the colony they eat. Look at the base and between polyps under magnification. This is the single most misdiagnosed cause of a declining zoa colony.
- Sundial snails — small, flat, patterned snails that eat zoas at night. Check the colony after dark with a light.
- Asterina starfish — usually harmless, occasionally a nuisance on zoas.
- Basic irritation — too much flow, too much light after a move, or a neighbour stinging them. Zoas close for boring reasons constantly.
If the polyps are closed but the tissue looks intact and there are no white pitted lesions, you probably do not have pox. You probably have a pest or an unhappy placement.
The approach that works
Step one: dip and inspect. A coral dip is the right first move regardless of what you think it is, because it clears the pests that mimic pox and gives you a look at what falls off. Zoas tolerate dipping well when you follow the times for the dip you are using. Blast the base and the mat with a baster while you are in there.
Step two: isolate. Move the affected colony out of contact with healthy ones if you can. Whatever this is, it spreads.
Step three: cut your losses early. The hardest and most useful advice with zoas: if a colony is going polyp by polyp, frag off the healthy portion, dip it, and move it. Reefers lose entire collections trying to save a dying mat. A few clean polyps somewhere else is a colony again in six months.
Beyond that, the treatment reefers reach for is an antibiotic dip or bath — furan-based products have the longest track record here. Do it in a separate container, never in the display, and be aware you are treating on a hobby-level diagnosis. Improve the basics at the same time: stable parameters, adequate flow, and no accumulated detritus sitting on the mat.
Get the ID right first
The reason zoa problems drag on is that pests, infection and irritation look similar from three feet away and completely different under a loupe.
Our Coral Pest ID & Treatment Guide covers the zoa-specific pests — nudibranchs, sundial snails, asterina — with photos at useful magnification and the treatment for each, across 29 pages. Instant PDF download.
Pair it with The Complete Coral Dipping Guide for the dip times zoas actually tolerate.