6 minute read · October 1, 2026
Aquarium algae: what each kind is telling you
Brown film, green dots, hair, red slime, green water and black tufts each point to a different cause. How to tell them apart in a Palm Beach County tank and what actually fixes each one.
Brown dust on everything
Diatoms. A tan film on the glass, sand and rock that wipes off with a finger. It shows up in nearly every new tank and after a big substrate change because it feeds on silicates in fresh sand and tap water. It usually burns itself out in a few weeks. If it keeps coming back in an established tank, the water source is the suspect, and in Palm Beach County that often means tap water going in where RO/DI water should.
Hard green dots on the glass
Green spot algae. Tiny circles that laugh at a sponge and need a blade. It likes strong light and low phosphate, so it is common on reef tanks that are otherwise very clean and on freshwater tanks sitting near a window. Scrape it on a schedule and shorten the photoperiod. It is the one algae that is more of a chore than a warning.
Try it on your tank
Green hair and green fuzz
Hair algae is a nutrient problem wearing a green wig. Nitrate and phosphate are high, usually from overfeeding, too many fish, a filter that has not been cleaned, or a water change schedule that slipped. Pulling it by hand helps for a week. Fixing it means testing, cutting the food, cleaning the filter and getting water changes back on schedule, then giving the clean-up crew a chance to keep up.
Red or dark slime
Cyanobacteria, called red slime in saltwater and blue-green algae in freshwater. It is a bacterium, not an algae, and it grows in sheets that peel off in one piece and smell like a swamp. It wants low flow and dissolved organics. More flow across the dead spots, a deep clean of the sand, and a hard look at the feeding usually clears it. Chemical cures exist and they treat the symptom.
Green water you cannot see through
Free-floating single-celled algae, mostly a freshwater and pond problem. Light plus nutrients, often after a filter was rinsed in tap water and the bacteria crashed. Water changes alone rarely win because the algae doubles faster than you can dilute it. A few days of darkness or a UV sterilizer clears it, and then the cause gets fixed so it stays clear.
Black tufts and bubbles
Black beard algae in planted tanks grows on leaf edges and driftwood where flow is weak and carbon dioxide swings. Steady CO2, better circulation and trimming the worst leaves beat it. In reefs, green bubble algae is a different animal, a sack that spreads when it pops, so it comes out whole, by hand, with the tank's water running through a filter sock. Bryopsis, the feathery one, needs a specific treatment and is worth a photo to Jason before trying anything.
The pattern underneath
Almost every algae comes down to light, nutrients and flow. A South Florida tank near a window gets more light than its owner thinks. A tank fed twice a day by two different people gets more nutrients than anyone admits. And a pump that has slowed with age moves less water than the day it was bought. Test the water, read the numbers and the algae will tell you which of the three it is.
The short version
Brown means new or silicates, green dots mean light, hair means nutrients, slime means flow, green water means both. Fix the cause or it comes back.