Reef tank algae identification — what is this and how do I fix it?

DiagnosticBrown, easily wiped off with a finger or magnet. Tank is <3 months old. Silicate >0.5 ppm.

FixDon't fight it. Diatoms exhaust available silica in 4–8 weeks and disappear on their own. If persistent: add cleanup crew (Nassarius snails, hermit crabs), source silicate-free RODI water.

DiagnosticStringy green strands, often on rocks. Nitrate >5 ppm or phosphate >0.05 ppm or both.

FixReduce nitrate AND phosphate (need both — limiting just one doesn't work). Manual pull during water changes. Add herbivore (Mexican turbo snails are the workhorse; tangs help if you have a 4+ ft tank).

DiagnosticRed, pink, or black mat that sheets across sand or rocks. Releases small bubbles when stirred. Slimy texture.

FixIncrease flow to the affected area (cyano hates flow). Reduce nutrients (phosphate especially). For severe cases: Chemiclean (active ingredient is erythromycin-derived) — works in 3–5 days but doesn't address root cause.

DiagnosticBrown snot-like mass with trapped bubbles. Stringy when pulled. Reappears within hours of removal. Tank often runs nitrate=0, phosphate=0.

FixRAISE nutrients (counterintuitive). Target NO3 5–10 ppm, PO4 0.05 ppm. Stop carbon dosing if you're doing it. 3-day blackout. UV sterilizer kills free-swimming cells. Microscope ID highly recommended — different dino species respond differently.

DiagnosticLooks like tiny green pine trees or feather-fronds, vivid green. Snails won't eat it. Spreads aggressively across rocks.

FixFluconazole (Reef Flux or human-grade) at 20 mg/gal, once. Most bryopsis dies in 7–14 days. Run carbon to remove after treatment. Black-out for 2 weeks won't kill it; manual pull is futile.

DiagnosticRound, fluid-filled green bubbles, single or in clusters. Pops releasing spores if damaged.

FixManual removal: carefully unscrew at the base, do NOT pop. Emerald crabs (Mithraculus sculptus) eat it but slowly. Severe outbreaks: pull affected rock and bleach.

DiagnosticStiff, often slightly chalky green mat. Hard to scrub off rock. Tank usually has trace element imbalance.

FixICP test — usually a high silicate, high iron, or unbalanced minor element. Manual removal of affected rocks, address the trace imbalance via water changes.