Controlling Dinoflagellates in a Reef Tank

Dinoflagellates bloom when nutrients bottom out. Learn to identify the brown snotty mats, why raising nitrate and phosphate is the counter-intuitive fix, and how UV, biodiversity and patience help.

Dinoflagellates, usually shortened to "dinos," are among the most frustrating pests in reef keeping. They are single-celled protists, many of them photosynthetic and mixotrophic, meaning they both photosynthesise and ingest prey. In an aquarium they appear most often during the maturation of a new system or in tanks that have been driven to very low nutrient levels, and they can be stubborn enough to make hobbyists tear systems down.

Identifying dinos

Dinos typically form brown to golden-brown stringy, snotty mats over sand, rock, powerheads and corals, frequently studded with trapped oxygen bubbles produced by photosynthesis. A practical field test: siphon some off and agitate it in a glass of water; within a few hours it tends to congeal back into a single mass. Definitive confirmation is microscopic, since the visible mat can be confused with diatoms or cyanobacteria.

Why they bloom: the counter-intuitive part

Unlike most nuisance algae, dinos thrive when nutrients are scarce. They commonly become problematic when conditions fall out of balance, especially when nitrate sits below about 2 ppm and phosphate below about 0.02 ppm, or when the microbial community is immature. Ultra-low-nutrient, over-cleaned tanks remove the competition that would otherwise crowd dinos out. This is why chasing zero nutrients can backfire.

Control strategy

Because dinos exploit a near-sterile, nutrient-starved system, the foundation of control is rebuilding balance rather than scrubbing harder.

• Raise nutrients deliberately: bring nitrate up into roughly the 10-15 ppm range and phosphate into roughly the 0.08-0.1 ppm range by feeding more or dosing, which both feeds competitors and discourages dinos.
• Boost biodiversity: add live phytoplankton and microbial diversity so beneficial algae and bacteria out-compete the dinos for resources.
• Run a UV sterilizer: a sized UV unit with adequate flow can knock down free-floating cells over a couple of weeks, though some strains respond poorly.
• Manual export plus blackout: siphon mats out daily and pair with a short blackout or reduced photoperiod to weaken photosynthesis.
• Use hydrogen peroxide cautiously: it can give temporary relief, but higher doses risk stressing light-sensitive corals such as Montipora.