ID guide

Descriptions, photos, and movies of the pest algae I see most often in home aquaria.

If you have a pest that is not shown here, please consider sending it to me. I can give you an ID, and I will add it to this guide if it helps others. If you are not sure whether you have a dinoflagellate at all, start with Is it a dino?

Dinoflagellates

New to all this? Start with what a dinoflagellate is. Pick the one that looks and moves like yours:

OstreopsisPointed oval; spins around a fixed pointvery toxic GambierdiscusRound; large and easy to seetoxic AmphidiniumOval, grooves at the top; glides along surfaces ProrocentrumOval with a notch at the top; barely moves Small unidentified dinoTiny round cells in mucus; look for a T OthersCoolia, Alexandrium, Akashiwo, Symbiodinium

These all live on surfaces, which favors flat cells, so under a microscope they look like similar discs or sesame seeds. Look for the point, the notch or grooves at the top, and above all how the cells move.

Cartoons, not to scale. Each card shows the cell from above, with a small side view underneath. Orange cards are the most toxic, so take extra care with those. To read the pictures, see anatomy of a dinoflagellate.

Looking for treatment help?

Once you know what you have, see my treatment notes and the Dinoflagellate Guide, a detailed hobbyist-written guide with its own photos and quick-reference stats for each type.

Not dinoflagellates

These three are often mistaken for dinos. See the page on non-dinos for more.

CyanobacteriaLong, thin filaments DiatomsGlassy and pointed or round; slow-moving CiliatesTiny hairs; fast, darting predators

A caution about identification

Identifying a dinoflagellate to species usually takes an electron microscope or DNA sequencing. From light-microscope photos I can usually name the genus, and that is what you need to decide how careful to be. I mark a name "sp." when I cannot say more.