Dinoflagellates
Single-celled organisms that bloom as brown, stringy, bubble-filled snot over sand and rock. This is the one on this list most capable of emptying a tank: several of the species involved are toxic, and an established bloom kills cleanup crew and can take corals and fish with it. It also has a habit of appearing in tanks that are being kept scrupulously clean.
Make sure that's what it is
Identification genuinely matters here, more than for anything else on this list, because the species behave differently and respond to different treatments. The four common in home tanks are Amphidinium, Coolia, Prorocentrum and Ostreopsis, and telling them apart needs a microscope — a 100x scope is a real prerequisite, not a nicety. The shared field signs are brown stringy mats holding oxygen bubbles that appear during the photoperiod and largely disappear overnight.
What it looks like
No photos of Dinoflagellates 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.
Stop stripping the nutrients
HusbandryWhy it works
This is the counter-intuitive one and it is the core of the modern treatment. Dinoflagellate blooms correlate strongly with bottomed-out inorganic nutrients, particularly phosphate at an undetectable 0 ppm. Many of the species involved are mixotrophic — they can feed on organics as well as photosynthesise — so a tank starved of nitrate and phosphate hands them a competitive advantage over the algae and bacteria that would otherwise crowd them out. Raising nutrients does not feed the dinos so much as it lets their competitors back in.
What to do
Get phosphate up and keep it above roughly 0.05 ppm for the entire treatment period, and get nitrate measurably off zero. Stop water changes for the duration unless nitrate or phosphate is actively harming the tank, since a water change dilutes the thing you are trying to restore.
UV sterilisation
HusbandryWhy it works
Several of the dinoflagellate species that bloom in reef tanks spend part of the day-night cycle free-swimming in the water column rather than settled on rock and sand. A UV steriliser kills what passes through it, so it removes exactly that mobile fraction — which is also the fraction that reseeds a surface you just cleaned. This is why UV is effective against the water-column species and much weaker against the ones that stay bound to the substrate.
What to do
Size the unit and the flow rate through it for sterilisation rather than clarification, and run it continuously. It pairs with the nutrient work above rather than replacing it.
Keeping it out
Do not move rock or livestock from a tank with an active bloom into another tank.