Algae ID

Is it a dino?

Brown or red slime in a reef tank is usually cyanobacteria, diatoms, or dinoflagellates. Look at it through a hand lens or microscope and follow the chart. New to this? Start with what a dinoflagellate is.

Brown or red slime in the tank? Look at it through a hand lens or microscope Long thin filaments? yes Cyanobacteria no Pointed, slow-moving,crystal-like cells? yes Diatoms no Separate round, oval orheart-shaped cells? no Something else:send a photo yes: likely a dino Watch how the cells move Spin arounda fixed point like a tetherball Ostreopsis Glide smoothlyon surfaces grooved top Amphidinium Barely move,or tumble big round pyrenoid Prorocentrum Other things to consider Large round cells, easyto see at low power Gambierdiscus? Tiny round cells thathardly move Small unidentified dino Orange boxes are the toxic ones. Know what you have before you start killing it.
Follow the arrows. Click any result for details.

How dinos move

Motion is the best clue, so take a video if you can.

Ostreopsis Spins around a fixed point glass or slide surface Amphidinium Glides smoothly along surfaces Prorocentrum Barely moves, or tumbles Not to scale
The three main benthic pests, and how they move. Motion is the quickest way to tell them apart, which is why video is so useful.

Not sure? Send me your photos or video and I will tell you what I think. And do not start killing a bloom until you know whether it is toxic: see the safety notes.

Hints from the tank

Dinos often form a brown, amber, or red-brown film over sand and rock that comes off in strings and traps bubbles. Cyano is more often a red, purple, or black mat in low-flow spots, and diatoms a dusty brown coat on new tanks. The tank alone cannot tell you, but it is a clue.

Brown film of algae over sand in a reef tank
Tank view of a dino outbreak: a brown film over the sand and rock. Image courtesy of a hobbyist.
Brown film with trapped bubbles on sand
Another tank view. Bubbles trapped in the film are typical of dino mats. Image courtesy of a hobbyist.

What tends to cause each one

DiatomsA new tank or stirred-up sand, with plenty of silicate. Often temporary. CyanobacteriaLow flow and high nutrients. DinoflagellatesVery low, often unmeasurable, nutrients; a new or sterile tank; few grazers.

A rule of thumb from the outbreaks I have seen, not a law.

Why "algae" is not a useful word

It is a bit like saying "flying things": flies, bats, birds, and airplanes all fly, but they are not related. Cyanobacteria, red algae, green algae, diatoms, kelp, and dinoflagellates are all called algae, are only distantly related, and need different fixes. That is why the first job is working out which kind you have. If I was giving advice on dealing with a pest in your vegetable garden, I would have very different advice for dealing with rabbits as opposed to elephants, even though both are mammals.

Finding a dino does not mean you have a dino problem

When I look at sand from reef aquariums under a microscope, I can usually find some dinoflagellates. They are a normal part of the community, they cause no trouble, and you cannot see them without a microscope. In the same way, people often send me algae mats that are mostly cyanobacteria, and I can often find a few dinos mixed in. I would not call that a dino problem.

What matters is what is making up most of the bloom, the thing you can actually see growing in your tank. Only worry about dinos when they are the main contributor.

For a detailed guide to dealing with a bloom once you know what it is, see the Dinoflagellate Guide (written by another aquarist) and my treatment notes.