Cyano vs Dinos: How to Tell Them Apart and What Fixes Each

At 9 in the morning the sand looks clean. By 3 in the afternoon there is a rusty film across it, with bubbles caught in it and a few strings lifting off the rock. You siphon it out, and the next day it is back. Before you buy anything, you need to know whether that film is cyano or dinos, because the fixes pull in opposite directions. The thing that clears one can feed the other.

Why it matters which one you have

Cyanobacteria are bacteria that photosynthesize. They are not algae at all, even though the hobby calls them red slime algae. Dinoflagellates are single-celled organisms with two tiny tails that let them swim. Both show up as a brown or red film, and both trap bubbles, which is why people mix them up.

The trouble is that the standard fixes conflict. Cyano gets better when you clean up detritus, add flow and keep up with maintenance. Dinos often get worse when you export harder, because many dino outbreaks start in a tank where nitrate and phosphate have dropped to zero. Run big water changes and fresh phosphate media on a dino tank and you can make it worse. A cyano medication does nothing to dinos, and a dino treatment does nothing to cyano. So the first job is the ID.

Cyano vs dinos at a glance

What to check	Cyano (red slime)	Dinos
Color	Red, maroon or purple most often. Can be green or black.	Brown, tan or golden
Texture	A smooth, slimy sheet	Stringy, snotty strands, or a loose dust on sand
Siphon test	Lifts off in one piece, like a skin	Breaks into strings and clouds the water
Bubbles	Often	Often, sometimes lots of them
Through the day	Stays about the same, day and night	Builds up under the lights, fades or lets go after dark
Smell	Earthy when disturbed	No reliable smell
Under a microscope	Still threads or chains of cells. Nothing swims.	Single cells, and many of them swim
Typical nitrate and phosphate	Anything. High, low or normal.	Often very low, near zero
Typical tank	Dead spots, detritus, heavy feeding, missed maintenance	Clean, new or heavily filtered tanks
What treats it	Maintenance and flow first. Cyano treatments such as ChemiClean or Red Cyano Rx.	Measurable nutrients, UV, a blackout, and dino treatments such as Dino X

Bubbles do not settle the question. Both trap them, because both photosynthesize. Color helps, but it is not proof, since cyano also comes in brown. Use two or three of the checks below together.

Four checks that tell them apart

1. The siphon test

Take a turkey baster or a siphon to a patch. Cyano comes up as a mat. You can often lift a whole palm-sized sheet in one pull. Dinos fall apart into strings and a brown haze that drifts and settles back. Diatoms, the third look-alike, are a dry-looking brown dust that wipes off glass with one pass and do not form strings or mats.

2. The day and night check

Look at the same spot first thing in the morning, before the lights come on, and again late in the afternoon. A film that is mostly gone at dawn and back by afternoon points to dinos. Many common dino types attach to surfaces under light and drift up into the water at night. Cyano does not cycle like that. A cyano mat that was there at 6 p.m. is still there at 6 a.m.

3. The microscope

This is the only check that settles it for sure. Scrape a little of the film into a drop of tank water on a slide and look at it under a basic microscope. Cyano shows still strands or chains of tiny cells. Dinos show separate round, oval or bean-shaped cells, and many of them move. If something is swimming, it is not cyano.

4. The test kit

Test nitrate and phosphate with a kit that reads low, such as the Red Sea Algae Control Pro or a Hanna ultra-low range phosphate checker. If both read zero and the film is brown and stringy, dinos move to the top of the list. Cyano can grow at any nutrient level, so a normal or high reading does not rule it out.

Our algae ID guide covers the other common nuisance algae, including hair algae, bubble algae and diatoms, if the film turns out to be neither.

What causes cyano

Cyano is in every tank. It takes over when it has an edge over the bacteria and algae that normally keep it down. The usual edges are:

• Dead spots: corners, the sand behind rockwork and any low point where flow never reaches. Cyano almost always starts in the low-flow areas.
• Detritus. Uneaten food and waste that settles and rots feeds it directly.
• Heavy feeding and slipping maintenance: a dirty filter sock or roller, a skimmer that has drifted out of tune, missed water changes.
• Nutrients out of balance in either direction. Cyano is common in tanks with high nitrate and phosphate, and it also shows up in tanks where nitrate has been driven to zero while phosphate is still around.

Lowering nutrients on its own has a poor record as a cyano cure. Fixing flow and detritus has a good one.

What causes dinos

Dinos tend to show up in tanks that are clean on paper. The common pattern is a tank where nitrate and phosphate have read zero for weeks, often because of strong phosphate media, carbon dosing, a big refugium, a new tank with little life in it yet, or a run of large water changes. With nothing else competing for the space, dinos move in. Long photoperiods and warm, still water help them along.

There are several kinds, and they do not all act the same. Some live mostly in the water and on rock and come and go with the light. One common kind lives in the sand and stays there day and night as a loose brown dust, and that kind is harder to clear. This is why the same treatment works quickly in one tank and barely touches another.

How to fix cyano, in order

1. Siphon it out. Pull every mat you can reach, daily if you have to. It comes off easily, and every sheet you remove is less to fight.
2. Kill the dead spots. Turn or add a powerhead so the patch where it started gets moving water. If the mat keeps coming back in the same spot, that spot needs flow.
3. Clean up the detritus source. Feed less, change filter socks and rollers on time, clean the skimmer neck, and blow out the rockwork with a baster during water changes.
4. Get nutrients into a normal range. If nitrate and phosphate are very high, bring them down. If both read zero, let them come up until they are measurable by feeding a little more or running the refugium light less.
5. Give it a few weeks. A small outbreak can clear in a week or two once the causes are fixed. A bad one takes longer. Beneficial bacteria products such as Brightwell MicroBacter7 may help the good bacteria win back the surfaces, but expect weeks, not days.
6. If it is smothering corals, treat it. ChemiClean (also in 6g and liquid) and Red Cyano Rx usually clear cyano in one or two rounds when used as directed. Siphon first. Add an air stone or point a powerhead at the surface, because these treatments can drop oxygen. Pull carbon out. After treatment, do the water change the label calls for. Read the label for the skimmer, because the brands disagree: Boyd says keep the skimmer running and watch it for overflow, BlueLife says turn it off and restart it after 24 hours.

A treatment is a reset, not a cure. If the dead spots and detritus are still there, cyano comes back.

How to fix dinos, in order

1. Confirm it is dinos. Use the microscope or the day and night check before you change anything. Everything below is the wrong move for cyano.
2. Stop stripping the water. Take phosphate media offline, ease off carbon dosing, and skip big water changes for now. Each of those drops nutrients further.
3. Bring nitrate and phosphate back up. Aim for at least 2 to 5 ppm nitrate and around 0.05 ppm phosphate, then hold them there. Feed more, or dose a nutrient supplement such as Tropic Marin Plus-NP, and test every few days so you can see the numbers move.
4. Run UV. A UV sterilizer kills the dinos that drift through it, which is why it works best on the kinds that lift off the rock at night. It does little for the kind that stays in the sand. Our filtration section carries UV units in several sizes.
5. Consider a blackout. Lights off and the tank covered for about three days knocks many outbreaks back. Most tanks ride out a few days of darkness, but bring the lights back slowly. The sand-dwelling kind can survive a blackout, so it is a help, not a guarantee.
6. Add competition. Bacteria products and copepods give dinos something to compete with. In a new tank, time and more life in the system often do more than any bottle.
7. Dino treatments, if the steps above stall. Dino X and its newer version Dino M target dinoflagellates. They do not treat cyano. Follow the label exactly. The maker’s directions call for dosing in the evening, running the skimmer, leaving carbon and phosphate media out, shortening the photoperiod, and moving urchins, sea hares and other grazers to another tank first.

Hydrogen peroxide is the last resort some reefers use on dinos. It can burn soft corals and kill the good bacteria you are trying to grow, so if you try it, start low. Our algae ID guide gives the starting dose.

What a cleanup crew can and cannot do

Nothing in a cleanup crew reliably eats cyano or dinos. We would rather say that plainly than sell you snails for a problem they will not fix. What a crew does is prevention. Snails and hermits eat leftover food and film before it rots into the detritus cyano feeds on. Cerith snails and nassarius snails keep the top of the sand bed turned over, which gives a mat less time to settle in. A tank with a working crew and a living sand bed also has more competition for dinos.

If your tank is short on crew, the Complete Clean Up Crew, Florida Keys, by Tank Size sizes the crew to your tank. One caution for dino tanks: if you dose Dino X or Dino M, the maker says to move urchins and sea hares out first. For the green algae a crew does eat, see what eats hair algae.

Mistakes that make it worse

Mistake	Why it backfires
Treating dinos with a cyano product	Cyano treatments target bacteria. Dinos are not bacteria, so nothing happens and you lose a week.
Big water changes and fresh phosphate media on a dino tank	Both drop nutrients further, which is often what let dinos in.
Dosing a cyano treatment without extra aeration	Oxygen can drop during treatment, and that stresses fish and inverts.
Treating cyano and changing nothing else	The dead spots and detritus are still there, so it comes back.
Chasing zero nitrate and phosphate to starve algae	Corals need some nutrients, and zero is where dinos do best.
Adding new corals and fish in the middle of a dino outbreak	New livestock adds stress and you cannot tell what is working. Wait until the tank has been clear for a few weeks.

FAQ

The Underwater Gardener is a reef aquarium store in Key West, Florida. The cleanup crew animals linked on this page are collected by our team in the Florida Keys. Livestock ships UPS Next Day Air on Tuesdays and Wednesdays.