Reef Tank Algae ID Guide: What's Growing and How to Beat It
Identify the eight most common reef tank algae — from diatoms to dinoflagellates — and learn the specific cause and fix for each, so you stop treating the wrong problem.
Every reef tank grows algae. That is not a failure — it is biology. Nuisance algae only becomes a problem when you misidentify it and fight it with the wrong weapon: dosing hydrogen peroxide at what turns out to be cyanobacteria, or doing big water changes that actually feed a dinoflagellate bloom. This guide walks through the eight algae (and algae impostors) you are most likely to meet, how to tell them apart at a glance, what each one is telling you about your water, and the specific fix that works for each.
Why Algae Shows Up in Reef Tanks
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All photosynthetic nuisance organisms need three things: light, nitrogen, and phosphorus. Your corals need the same three things, which is why "starve the tank to zero" almost always backfires. The real goal is balance — nitrate around 2–10 ppm and phosphate around 0.02–0.1 ppm for most mixed reefs — plus enough competition and grazing pressure that nuisance species never get a foothold. If your numbers drift outside those bands, check them against our reef tank parameters chart before you touch anything else.
Two other factors matter more than most hobbyists realize. First, tank maturity: a reef under 12 months old simply does not have the biodiversity to outcompete pioneer species, so diatoms, film algae, and even dinoflagellates are close to inevitable in year one. Second, source water: tap water can carry 0.1–1.0 ppm phosphate and measurable silicate, which is a standing invitation to diatoms and hair algae. If you are still mixing salt with tap or questionable RO water, a proper 4-stage unit producing 0 TDS is the single highest-leverage upgrade you can make — Check price on Amazon.
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Diatoms: The Brown Dusting on New Tanks
What it looks like: A light-brown to golden powder coating the sand, rock, and glass, usually appearing 2–6 weeks after the cycle finishes. It wipes away with almost no resistance and often looks worse by evening than it did in the morning.
What causes it: Silicate. New sand, new rock, and unfiltered top-off water all leach silica, which diatoms (Navicula and friends) use to build their glass shells. Diatoms are almost always the very first thing to bloom in a new tank.
How to beat it: Mostly, wait. Diatoms burn through the available silicate in 2–4 weeks and fade on their own. Speed things up with Trochus or Cerith snails (roughly one per 2 gallons), keep top-off water at 0 TDS, and siphon the sand surface during water changes. Do not panic-dose anything — this one resolves itself.
Green Hair Algae: The Classic Nutrient Problem
What it looks like: Bright green filaments, from short turf to 3-inch flowing strands, usually anchored to rock. Under magnification, Derbesia (true green hair algae) looks like unbranched or sparsely branched threads.
What causes it: Sustained elevated nutrients — typically nitrate above 25 ppm or phosphate above 0.15 ppm — combined with light. Overfeeding, an overstocked tank, dying rock, or a skimmer that has quietly stopped working are the usual suspects.
How to beat it: Manual removal first, always. Pull as much as you can with your fingers or a toothbrush during a water change, because every strand you remove is nutrient export. Then fix the source: cut feeding by a third, clean the skimmer, and follow the export strategies in our guide on how to lower nitrates. Stock grazers that actually eat hair algae — Mexican Turbo snails, tuxedo urchins, and (in tanks over 50 gallons) a foxface or bristletooth tang. Expect 4–8 weeks from intervention to a clean tank; anyone promising faster is selling something.
Bryopsis: The Hair Algae That Ignores Your Clean-Up Crew
What it looks like: Dark green, feather-shaped fronds with a distinct central stem and side branchlets — like tiny ferns. It feels wiry, not slimy, and grazers refuse to touch it.
What causes it: Bryopsis is an opportunist that arrives on frags and live rock. It grows in low-nutrient tanks too, which is how you distinguish it from Derbesia: if your nitrate is 5 ppm and the "hair algae" keeps spreading while snails ignore it, it is Bryopsis.
How to beat it: Nutrient control barely touches it. The reliable fix is elevating magnesium to 1500–1600 ppm for 2–3 weeks using a magnesium supplement (Kent Tech-M has the best track record, likely due to a trace impurity), or treating with fluconazole at 20 mg per 10 gallons, which typically melts Bryopsis in 7–14 days. Remove GFO and carbon during fluconazole treatment and skim wet afterward.
Cyanobacteria: The Red Slime Impostor
What it looks like: Sheets or mats of deep red, maroon, or occasionally dark green slime, often with trapped bubbles, coating the sand and low-flow rock. It peels off in satisfying rubbery sheets and comes back within 48 hours.
What causes it: Cyano is a bacterium, not an algae, and it thrives on imbalance rather than sheer nutrient load. Classic triggers: nitrate at 0 while phosphate stays measurable, dissolved organics from overfeeding, old bulbs shifted toward red spectrum, and dead spots with poor flow.
How to beat it: Siphon the mats out directly, increase flow across affected areas, and rebalance nutrients — if nitrate reads 0, that is part of the problem, and dosing nitrate to 2–5 ppm often clears stubborn cyano within weeks. Cut feeding of powdered coral foods temporarily. As a last resort, chemiclean-style antibiotics work but treat the symptom, not the cause; without fixing flow and nutrient balance, it returns in a month.
Dinoflagellates: The One That Punishes Clean Tanks
What it looks like: Brown to amber snot with air bubbles, forming stringy strands off the rock and sand. The tell: it nearly vanishes overnight and rebuilds by mid-afternoon, because dinos are motile and migrate into the water column in darkness. Some species (Ostreopsis) are toxic enough to kill snails that graze them.
What causes it: Bottomed-out nutrients. Dinos exploit tanks where nitrate and phosphate both read zero — usually after aggressive GFO use, carbon dosing, or an over-sized refugium strips the water and starves out the competing microbial community.
How to beat it: This is the one bloom where you should stop water changes temporarily and raise nutrients: dose to keep nitrate at 5–10 ppm and phosphate at 0.05–0.1 ppm. Run a UV sterilizer (for Ostreopsis-type free-swimming species), blackout the tank for 72 hours for photosynthetic mats, and add biodiversity back with live phytoplankton, copepods, and a cup of sand from a mature tank. Identify your species with a cheap USB microscope first — treatment differs by type. If you cannot figure out why your nutrients keep crashing, a full elemental workup will usually find the culprit — Check price on Amazon.
Green Film and Green Spot Algae on the Glass
A thin green film that returns to the glass every 2–3 days is completely normal in a healthy reef — it is the price of keeping nutrients high enough for corals. Green spot algae (hard, coin-sized dots that laugh at your fingernail) shows up on glass in high-phosphate tanks and needs a metal blade to remove. A good magnetic cleaner with a scraper attachment turns this from a weekly chore into a 90-second habit — Check price on Amazon. Scrape acrylic tanks with plastic blades only.
Bubble Algae and Turf Algae: The Hitchhikers
Bubble algae (Valonia) arrives as glossy green marbles on frag plugs and rock. Small numbers are cosmetic; the problem is popping them carelessly, which can release spores. Twist whole bubbles off with the surrounding attachment point, ideally with a siphon running next to your hand. Emerald crabs (Mithraculus sculptus) eat Valonia with reasonable reliability — stock one per 20–30 gallons.
Turf algae forms short, wiry, carpet-like patches in mixed brown-green-red tufts that grip rock like Velcro. It is the hardest algae on this list to eradicate: manual scrubbing with a wire brush (on rock removed from the tank where possible), sea hares for large outbreaks, and relentless grazing pressure from urchins are the realistic playbook. Expect months, not weeks.
Reef Tank Algae Quick-Reference Table
| Algae type | Key identifier | Root cause | Primary fix |
|---|---|---|---|
| Diatoms | Brown dust, wipes off easily | Silicates in new tanks/water | Wait 2–4 weeks; 0 TDS top-off; snails |
| Green hair (Derbesia) | Green filaments, grazers eat it | NO3 >25, PO4 >0.15 | Manual removal + nutrient export + grazers |
| Bryopsis | Fern-like fronds, grazers refuse it | Hitchhiker, any nutrient level | Fluconazole or Mg to 1500–1600 ppm |
| Cyanobacteria | Red rubbery mats, peels in sheets | Nutrient imbalance, low flow | Siphon, add flow, rebalance NO3:PO4 |
| Dinoflagellates | Snotty brown strings, gone at night | NO3 and PO4 at zero | Raise nutrients, UV, biodiversity |
| Green spot | Hard green dots on glass | Elevated phosphate | Metal scraper; lower PO4 |
| Bubble algae | Green marbles | Hitchhiker on frags | Careful removal; emerald crabs |
| Turf algae | Wiry Velcro-like carpet | Mature infestation | Scrub, urchins, sea hare, patience |
Building an Algae-Resistant Reef
Winning against algae long-term is about systems, not products. Four habits do most of the work:
- Control what goes in. 0 TDS RO/DI water, measured feeding (everything eaten in under two minutes), and quarantining or dipping frags so Bryopsis and Valonia never enter.
- Keep export running. A properly tuned skimmer, regular water changes, and either a refugium with chaetomorpha or a small GFO reactor give nutrients somewhere to go besides algae.
- Stock grazers before you need them. A clean-up crew of Trochus, Cerith, and Nassarius snails plus an urchin in larger tanks applies constant pressure, so blooms never get past the invisible stage.
- Test on a schedule. Weekly nitrate and phosphate readings catch a trend three weeks before your eyes do.
We keep our current recommendations for skimmers, test kits, and RO/DI units on our gear picks page. And if you are still in your first year and every week brings a new brown or green surprise, that arc — cycle, diatoms, ugly stage, stability — is exactly what our ebook The First Tank walks you through month by month.
FAQ
How do I tell dinoflagellates apart from diatoms? Timing and texture. Diatoms are a flat powder that wipes away and stays gone for a day; dinos form snotty, stringy strands with trapped bubbles that shrink dramatically overnight and rebuild by afternoon. A $30 USB microscope settles it definitively — diatoms look like geometric glass boxes, dinos like swimming ovals.
Will a 3-day blackout kill my algae without hurting corals? Most corals tolerate a 72-hour blackout with no lasting harm, and it is genuinely effective against cyanobacteria and photosynthetic dinoflagellates. It does very little against hair algae, Bryopsis, or turf algae, which simply resume growing when the lights return. Use blackouts as one tool alongside nutrient correction, never as the whole plan.
Should I run my nutrients at zero to prevent algae? No — zero nitrate and phosphate is the classic setup for a dinoflagellate or cyanobacteria outbreak, and it starves corals too. Target roughly 2–10 ppm nitrate and 0.02–0.1 ppm phosphate. Algae problems at those levels are almost always a grazing, flow, or hitchhiker issue rather than a nutrient one.
How long does the ugly stage last in a new reef tank? Most tanks cycle through diatoms, film algae, and some combination of cyano or hair algae between months 1 and 8, with noticeable stabilization by month 10–12 as coralline algae and microfauna diversity build. You can shorten it with 0 TDS water, restrained feeding, and early clean-up crew stocking, but you cannot skip it entirely — maturity is the cure.
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