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Green Hair Algae (GHA)
Derbesia spp. / Cladophora spp.
Stringy, bright-green, hair-like tufts that wave in the current. Grows on rocks, glass, and overflows. Easy to grab between fingers. Can carpet a tank quickly when nutrients are elevated.
Common Causes
- Elevated nitrate (above ~10 ppm) and/or phosphate (above ~0.1 ppm)
- Excess organics from overfeeding or under-cleaned filter socks
- Old reef bulbs - lights past their PAR shift fuel nuisance algae
- Poor water flow allowing detritus to settle
Treatment Steps
- Manually rip out as much as possible during a water change - get it before it sporulates
- Test nitrate and phosphate. Reduce feeding, add GFO/biopellets, or run carbon dosing to bring numbers down
- Run a refugium with chaeto on a reverse light cycle to compete for nutrients
- Add a hungry CUC (see below) and consider 1-2 day blackouts to weaken the bloom
- Replace reef bulbs if older than 12 months and audit photoperiod (most reefs need only 8-9 hours)
Biological Controls
Mexican turbo snails, trochus snails, emerald crabs, sea hares (for big outbreaks - rehome after), and yellow/sailfin tangs.
Feathery, dark-green pinnate fronds that look like tiny ferns or pine needles. Often confused with GHA but the structure is rigid and fern-shaped, not hair-like. Spreads aggressively from a single tuft.
Common Causes
- Hitchhiked in on a frag, live rock, or dry rock that wasn't cleaned
- Imbalanced major elements - especially low magnesium relative to calcium and alkalinity
- Once established, it self-perpetuates regardless of nutrients
Treatment Steps
- Raise magnesium to ~1600 ppm using Tech-M, Brightwell Magnesion-P, or equivalent magnesium chloride product. Hold for 4-6 weeks
- Manually pluck visible bryopsis - but expect it to come back if you skip the magnesium dose
- Spot-treat stubborn patches with Vibrant or hydrogen peroxide dipping (out-of-tank rock dips only)
- Monitor calcium and alkalinity - elevated Mg can shift these, so test more often
- Most CUC will not eat bryopsis - this is a chemistry fight, not a janitor problem
Biological Controls
Lettuce sea slugs (Elysia crispata) eat it but die when it's gone. Emerald crabs occasionally pick at it. Tangs almost universally ignore it.
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Bubble Algae
Valonia ventricosa / Ventricaria
Round, shiny, bright-green vesicles (water-filled bubbles) ranging from BB-sized to grape-sized. Grow individually or in clusters in rock crevices. Pop releases spores that seed new bubbles.
Common Causes
- Hitchhiked in on rock or frag plugs
- High nutrients accelerate growth but bubble algae appears in clean tanks too
Treatment Steps
- Remove rocks one at a time during water changes - twist bubbles off intact at the base, do not pop
- If a bubble pops in-tank, siphon the area immediately to limit reseeding
- For rock you can pull, scrub the holdfast with a stiff brush after removal
- Add emerald crabs (Mithraculus sculptus) - they target bubble algae specifically
- Spot-treat with hydrogen peroxide via syringe on rock that's out of the tank only
Biological Controls
Emerald crabs (1 per 20-30 gallons). Most tangs and angels ignore it. Foxface and naso tang occasionally graze.
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Cyanobacteria (Red Slime)
Cyanophyta - bacteria, not true algae
Reddish-purple, magenta, or burgundy slimy sheets covering sand, rock, and overflows. Often has tiny oxygen bubbles trapped in the film. Can be peeled off in mats. Sometimes appears blackish-green or rusty depending on the strain.
Common Causes
- Low water flow - dead spots collect detritus and let cyano colonize
- High organic load relative to flow - heavy feeding, light filter sock changes
- Imbalanced N:P ratio - very low nitrate with phosphate present is a classic trigger
- Old or dirty reef lights with degraded spectrum
Treatment Steps
- Increase flow with extra powerheads aimed at affected areas - cyano cannot establish in turbulent water
- Siphon out visible mats during water changes - do not just blow them around
- Clean or replace filter socks every 3-5 days; rinse the skimmer cup weekly
- Audit your nutrient ratio - if nitrate is undetectable and phosphate is present, raise nitrate slightly with sodium nitrate dosing or feed more
- Last resort: Chemiclean or Red Cyano Rx - works fast but only buys time if husbandry isn't fixed
Biological Controls
Cerith snails and nassarius snails help by stirring sand. No CUC reliably eats cyano.
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Dinoflagellates
Ostreopsis / Amphidinium / Coolia / Prorocentrum
Brown, snotty, stringy strands with abundant trapped bubbles. Has a daily cycle - usually peaks in mid-afternoon, recedes overnight. Coats sand, rocks, and corals. Often slimy when touched. Multiple species look slightly different (mucus-y vs sandy vs hair-like).
Common Causes
- Ultra-low nutrients - nitrate at zero and phosphate at zero is dino paradise
- Aggressive carbon dosing or GFO that crashes nutrients
- Loss of microbial diversity - new sterile dry rock tanks are very prone
- Hitchhiking from another infested system
Treatment Steps
- Stop chasing zero. Raise nitrate to 5-10 ppm and phosphate to 0.05-0.1 ppm by feeding more, dosing potassium nitrate, or pausing GFO/carbon dosing
- Run a 3-day full blackout (cover the tank with a blanket) - skip if SPS-heavy and consult coral tolerances first
- Add a UV sterilizer sized correctly for the tank volume (turnover rate matters more than wattage)
- Dose Dr. Tim's Waste-Away or a similar bacterial blend to rebuild microbial diversity
- Add a refugium with chaeto and pods - biodiversity is the long-term cure
Biological Controls
Most CUC AVOID dino-covered surfaces (it's toxic to many). Pods and microbial diversity from a refugium do the real work.
Brown or rust-colored dust coating sand, rocks, and glass in a uniform layer. Wipes off easily but quickly returns. Most common in tanks under 6 months old.
Common Causes
- Silicate leaching from new dry rock and sand - normal cycling phenomenon
- Tap water with silicates - always use RO/DI
- New tank instability before nutrient export pathways mature
Treatment Steps
- Wait it out. Diatoms self-resolve in 4-8 weeks as silicate is consumed and the tank matures
- Wipe glass with a magnet and stir the sand bed at water changes
- Confirm RO/DI water is producing 0 TDS - replace cartridges if not
- Add nassarius and cerith snails to keep sand clean while it passes
Biological Controls
Trochus snails, cerith snails, nassarius snails, and most hermits mow through diatoms.
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Coralline Algae
Corallinaceae - calcareous red algae
Pink, purple, magenta, or red HARD encrusting crust on rocks, glass, and equipment. This is BENEFICIAL - it indicates stable parameters and a healthy reef. Do not remove it.
Common Causes
- Stable calcium (380-450 ppm), alkalinity (8-10 dKH), and magnesium (1300-1450 ppm)
- Adequate magnesium is the unsung hero of coralline growth
- Moderate light - too intense actually slows it
Treatment Steps
- Leave it alone. This is the algae you WANT.
- Scrape it off the front viewing pane only - leave it everywhere else
- If you want more, dose 2-part or run a calcium reactor and keep magnesium at 1350+ ppm
- Coralline can fade under low magnesium or unstable alkalinity - check chemistry if it's receding
Biological Controls
Don't remove. If excessive on glass, scrape with a razor.
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Turf Algae
Mixed filamentous species
Short, stiff, dark-green carpet that grips the rock like AstroTurf. Less than 1 cm tall but extremely tenacious. Often appears alongside coralline and is hard to scrape off.
Common Causes
- Persistent low-grade nutrient excess
- Old lighting and long photoperiods
- Once established it has deep holdfasts that survive most CUC grazing
Treatment Steps
- For affected rocks you can pull: scrub with a stiff brush in old tank water, or do an acid bath (citric or muriatic) to strip everything
- For rocks that must stay in: spot-treat with hydrogen peroxide via syringe outside the tank during water changes
- Run carbon dosing or a refugium to lower nitrate and phosphate
- Trim photoperiod by 1-2 hours and replace bulbs if older than 12 months
- Be patient - turf takes weeks of consistent pressure to fully recede
Biological Controls
Yellow tang, sailfin tang, kole tang, and foxface will graze it. Urchins are the heavy artillery (pencil urchin, tuxedo urchin) but watch them around corals.
Various structured forms - grape-cluster Caulerpa racemosa, ferny Caulerpa taxifolia, blade-like Caulerpa prolifera. Bright green, clearly differentiated leaf structure. Can be intentional (refugium nutrient export) or invasive (display).
Common Causes
- Often added intentionally to refugiums
- Hitchhiking from frag plugs or live rock when unwanted
Treatment Steps
- If in a refugium: harvest aggressively and prune before it goes sexual (sporulation event releases gametes that crash water clarity)
- If unwanted in display: pull root system completely - any holdfast left will regrow
- Switch refugium algae to chaetomorpha if Caulerpa keeps sporulating - chaeto never goes sexual
- If a sporulation event happens, run carbon, change water, and check fish for stress
Biological Controls
Foxface, lawnmower blennies, and some tangs will graze it. Most reefers prune manually.
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Lobophora / Encrusting Brown Algae
Lobophora variegata
Brown leathery sheets and fan-shaped lobes encrusting rocks and overflowing onto corals. Smooth, glossy when wet. Extremely tough - cannot be scrubbed off easily.
Common Causes
- Hitchhiking from frags or live rock - very hard to spot in early stages
- Persists across a wide range of nutrient profiles, so chasing parameters rarely works
Treatment Steps
- Manual removal of affected rock + acid bath or stiff-brush scrub out of tank
- Spot-treat with hydrogen peroxide outside the tank for any rock you pull
- Sea hares can dent a major outbreak fast - rehome them once gone
- If it's overgrowing corals, frag the affected colony and remount on a clean plug
- Lobophora is a long-haul fight - expect months of pressure
Biological Controls
Sea hares (Aplysia) are the most effective biological control. Most tangs and angels ignore it.
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Green Film Algae
Mixed unicellular green algae
Light green coating on the front and side glass. Soft, slimy, easy to wipe off with a magnet. Returns within a few days. Considered normal in any healthy reef.
Common Causes
- Normal byproduct of available light + nutrients - every reef has some
- Excess if nutrients are very high or photoperiod is long
Treatment Steps
- Use a magnet cleaner weekly on the viewing pane
- Don't worry about it on the back glass - it grows there too and is harmless
- If it covers the glass within a day, audit nutrients and feeding
Biological Controls
Trochus and astrea snails graze film algae continuously. Most tanks balance with just a few snails per 20 gallons.
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Brown Jelly Disease
Protozoan infection (often Helicostoma)
NOT actually algae - a protozoan infection that often gets confused with algae. Brown gelatinous mass spreading rapidly across an LPS coral (frogspawn, hammer, torch, scolymia). Eats coral tissue overnight. Fast death without intervention.
Common Causes
- Stress events - shipping, temperature swings, parameter changes
- Damage to coral tissue followed by opportunistic protozoan infection
- Spreads to neighboring corals if untreated
Treatment Steps
- Pull the affected coral immediately and isolate in a separate container
- Turkey-baster blast off the brown jelly under a strong stream of tank water
- Dip the coral in a coral iodine dip (Bayer, Coral Rx, or povidone-iodine) for 15 minutes
- Frag healthy tissue away from infected tissue if necessary - cut clean
- Return only fully clean tissue to the display, observe daily for recurrence
Biological Controls
No biological control - this is a medical issue. Speed of response is everything.