Bryopsis

Bryopsis

NuisanceUnsightly and competes for space, but won't kill livestock on its own.

A dark-green algae growing in flat, feather- or fern-shaped fronds off rock and frag plugs. It will not kill anything directly, but almost nothing eats it — most of the cleanup crew that handles hair algae will walk past Bryopsis — so it does not resolve on its own the way a normal algae bloom does.

Make sure that's what it is

Look at the frond shape under a light. Hair algae is filamentous and unstructured; Bryopsis has a distinct central stalk with paired branchlets coming off it, like a tiny fern or feather. If a tang, urchin or snail has been ignoring it for weeks while grazing everything around it, that is the other tell.

What it looks like

No photos of Bryopsis yet. If you have it in your tank, a clear shot here is what the next person will compare theirs against — it's the most useful thing you can add to this page.

Getting rid of it

In the order we'd try them. Every method cites where it comes from, and where the reason a treatment works isn't actually established, it says so.

Fluconazole — effective, and not understood

Chemical

Why it works

Stated plainly because most sources do not: fluconazole is widely reported to clear Bryopsis, and the mechanism by which it does so is NOT established. It is a triazole antifungal that inhibits lanosterol 14-alpha-demethylase in fungi, and the popular explanation extends that to an ergosterol pathway in Bryopsis — but that extension is inferred, not demonstrated. What is actually known here is an observed effect without a confirmed cause, and that is a materially different thing to act on.

What to do

Dosed to the whole system over a period of weeks, with carbon and any chemical filtration removed while it is in the water. Skimming is usually reduced. Follow the specific product's instructions rather than a forum number.

Careful: This is not a benign treatment. A 2024 write-up documents reports of SPS colour loss and one case of losing nearly an entire SPS collection, and raises a plausible concern about the endolithic fungi corals rely on for sulphur cycling and immune response. Treat it as a last resort after nutrient export and manual removal, not a first move — which is the linked source's own recommendation.

Source: Does Dosing Fluconazole Have Negative Consequences? (Reef Builders, 2024)