Reef Tank Algae Identification: A Visual Guide

Nuisance growth is the most common reason reef keepers go looking for answers, and it is also the easiest problem to treat incorrectly — because the five most common culprits look superficially similar but have almost opposite causes. Scrubbing off a bloom without identifying it first often means it is back within days. This guide is a quick visual reference for telling the main types apart, what each one is actually telling you about your water, and the first step to take once you know what you are looking at.

A quick visual ID chart

Diatoms: brown dusty film

Diatoms are single-celled algae with a silica shell, and they are usually the very first growth a new reef tank produces — a soft brown dust coating the glass and sandbed within the first few weeks. They feed on dissolved silicate, which leaches out of sand, rock and tap water during a tank's early months. A diatom bloom in a new setup is normal and typically fades on its own within four to eight weeks as silicate levels settle and the microbiome matures. A bloom that reappears in an established tank is more often a sign of a silicate source such as unfiltered top-off water.

Green hair algae and bryopsis

Green hair algae (GHA) is the classic nuisance algae: bright green strands that carpet rock and substrate when nitrate and phosphate run persistently high relative to what the tank's corals and clean-up crew can use. It responds fairly quickly to reducing the nutrient surplus — cutting back feeding, checking for a dead spot behind rockwork, and exporting nutrients through water changes or carbon dosing.

Bryopsis looks similar at a glance but is a different, tougher problem. Its feathery, almost fern-like fronds regrow from microscopic fragments left behind after manual removal, which is why scrubbing alone rarely works and can make an outbreak worse by spreading it. Bryopsis is strongly associated with elevated phosphate that has been allowed to sit high for an extended period, so the fix is the same nutrient-export approach as GHA, just with more patience and careful, thorough removal rather than casual scrubbing.

Cyanobacteria (red or green slime)

Cyanobacteria forms a smooth, often slightly slimy sheet — most commonly red or maroon, sometimes green — that peels away from the sand or rock in one continuous film rather than leaving individual strands behind. It thrives where nutrients are elevated and flow is weak, which is why it tends to appear first in low-flow corners and behind rockwork before spreading. Improving flow to those dead spots and bringing nutrients back into a normal range are the two levers that matter most; sheet-forming blooms that keep returning almost always trace back to one of those two conditions still being present somewhere in the system.

Dinoflagellates: the low-nutrient exception

Dinoflagellates are the one entry on this list caused by the opposite problem. They appear as brown, snotty strands that trap oxygen bubbles and visibly pearl and lift off the sandbed under light — and they take over specifically when nitrate and phosphate have dropped to an undetectable level, starving out the competing bacteria and microalgae that would otherwise keep them in check. Treating a dino outbreak by cutting nutrients further, the instinct that works for hair algae and cyano, makes it worse. If bubbles and pearling are part of what you are seeing, the fix runs in the opposite direction — see our full guide to dinoflagellate recovery for the step-by-step approach.

First steps once you know what you're looking at

Before reaching for a scrubber or a bottled treatment, test nitrate and phosphate and note which side of the ideal range you are on. High-nutrient growth — GHA, bryopsis, cyano — calls for trimming the nutrient surplus and, for cyano, improving flow. Low-nutrient growth — dinoflagellates — calls for the opposite: feeding more and easing off nutrient export until levels recover to a detectable range. Diatoms are usually self-limiting and mostly need time. Whichever type you are dealing with, log the nutrient readings alongside a note or photo of the coverage every few days. That is what turns "it looks a bit better, maybe" into an actual trend line showing the surplus or deficit closing — and it is the fastest way to catch a treatment that is not working before you have wasted weeks on it.