A small amount of algae in a healthy aquarium is normal. It's when it takes over that you have a problem — and the problem is almost never "too much algae." It's an imbalance in light, nutrients, or both.
Scraping algae off the glass without fixing the underlying cause is like bailing a leaking boat. You can keep doing it, or you can fix the leak. Here's how to figure out what type of algae you have and what's actually driving it.
Brown algae (diatoms)
Brown, dusty film coating everything in a new tank. This is diatom algae, and it's almost universal in tanks less than three months old. Diatoms thrive on silica, which is abundant in most tap water and new substrate. They outcompete other algae early in a tank's life but tend to disappear on their own as the tank matures and silica levels drop.
Don't stress about diatoms. Otocinclus catfish and nerite snails eat them enthusiastically. If you have neither, just wipe the glass every week or two and wait it out. They almost always go away.
Green spot algae
Hard green dots on the glass and broad-leafed plants. Very common. Usually caused by low phosphate levels — counterintuitively, algae that forms hard spots tends to appear when phosphate is very low rather than high. In planted tanks this often coincides with insufficient fertilization.
Nerite snails are the best solution. They're one of the few animals that will actually eat green spot algae. Mild cases can be scraped with a magnetic algae cleaner or credit card. If it's persistent, check your phosphate levels and consider adding a fertilizer with phosphate if you're running a planted tank.
Green hair algae
Long, filamentous green algae that grows in tufts on hardscape, plants, and equipment. The most common algae complaint in the hobby. Causes: too much light, too many nutrients (especially nitrate and phosphate), or both.
Fix the light first. Reduce photoperiod to 6–7 hours and see if growth slows over two weeks. If the tank has no live plants, reduce intensity as well. Then address nutrients through more frequent water changes and not overfeeding.
Amano shrimp eat green hair algae aggressively. A group of 10 in a 30-gallon tank can make a visible dent within a week. Siamese algae eaters (the real ones, not flying foxes which are often mislabeled) also eat it.
Blue-green algae (cyanobacteria)
Technically not algae — cyanobacteria is a bacteria that photosynthesizes. It forms a slimy, blue-green or reddish-brown film over substrate and plants with a distinctive musty smell. Unlike true algae, almost nothing eats it.
Cyanobacteria thrives in low-flow areas with high nutrients and low nitrate relative to phosphate. The fix is increasing circulation to eliminate dead spots, improving nutrient export through water changes, and sometimes a targeted antibiotic treatment (erythromycin works but disrupts the tank's nitrogen cycle). A 3-day blackout — completely covering the tank for 72 hours — can knock it back enough to remove manually.
Black beard algae (BBA)
Dark grey or black tufts that grow on plant edges, hardscape, and equipment intakes. Very difficult to remove once established. BBA is strongly associated with CO2 fluctuations — tanks that run CO2 on a timer and have large swings between daytime and nighttime CO2 levels tend to develop it.
Spot-treating with Excel (liquid carbon) directly on dry BBA during a water change kills it effectively — turn off the filter, lower the water level, spray Excel directly on affected areas, wait 5 minutes, then refill. The dead BBA turns red and can be removed or eaten by fish. Siamese algae eaters will eat dead BBA.
Long-term, stable CO2 levels are the real fix. If you're running pressurized CO2 and have BBA, check that your CO2 isn't fluctuating too much overnight.
The nuclear option: blackout
A 3–5 day complete blackout — tank covered with a blanket, no light whatsoever — will kill or severely weaken most algae without harming fish. Plants will lose some color and may have some die-back but generally recover within a week. Feed sparingly during the blackout, don't open the tank, and do a large water change and gravel vacuum when it's over.
Blackouts work best as part of a fix, not a standalone solution. If you blackout and then return to the same conditions that caused the algae, it comes back within weeks.
The actual solution
Balance. Light matched to plant mass and type. Nutrients matched to plant consumption. Good flow so there are no stagnant areas. Regular water changes. Not overfeeding. A few algae-eating animals as a cleanup crew.
No single fish or product eliminates algae. The tank that doesn't have algae problems is one where conditions don't favor algae overgrowth in the first place.