Beating algae: identify it, then fix the cause

Short answer: algae is a symptom, not the disease. It takes hold when light and nutrients outrun what your plants and clean-up crew can use. To beat it for good, work out which type you have, then fix the cause — usually too much light, too long a photoperiod, or excess nutrients from overfeeding and overstocking. Scrubbing the glass and adding algae eaters both help, but only once you have brought light and nutrients back into balance. Treat the cause and the algae fades; treat only the symptom and it comes straight back.

First, identify what you are looking at

Different algae point to different causes:

• Brown, dusty film (diatoms) — extremely common in the first weeks of a new tank, and usually fades on its own as the tank matures. See how to cycle a new aquarium.
• Green spot and green dust — hard green dots on glass and slow-growing leaves; often tied to strong or long lighting.
• Green water — a pea-soup bloom in the water column, driven by light and nutrients, often too much sunlight on the tank.
• Hair and thread algae — long green strands, usually a sign of excess light and nutrients together.
• Black beard algae — dark tufts on edges and hardscape; stubborn, and often linked to unstable conditions and low flow.

Test before you treat

Guessing wastes weeks. A liquid test kit tells you what your nutrients are actually doing — a persistent algae problem often traces back to nitrate or phosphate quietly climbing. The API Master Test Kit covers the day-to-day parameters, and a phosphate test is worth having when algae will not explain itself.

Fix the cause, not the symptom

Most algae problems come down to a few levers:

• Light — shorten the photoperiod to around six to eight hours on a timer, and keep the tank out of direct sunlight. If your fixture is very bright over a low-tech tank, dimming or raising it helps. Browse lighting and canopies if yours has no timer or control.
• Feeding — overfeeding is the most common hidden nutrient source. Feed less, and remove uneaten food.
• Maintenance — regular water changes export the nutrients algae feeds on. See aquarium water changes.
• Nutrient control — where phosphate is genuinely high, a phosphate-removing medium such as Seachem PhosNet can help while you get the source under control.

Put a clean-up crew to work

Once the balance is right, algae eaters keep things tidy — they are the finish, not the fix. Nerite snails graze film algae and do not breed in fresh water, shrimp pick at soft algae and biofilm, and various algae eaters earn their keep. See our snails and shrimp for the workers most keepers rely on.

Manual removal helps too

For glass, a magnet cleaner such as the Mag-Float clears green spot without getting your hands wet, and pulling out hair algae by hand at water-change time removes a lot of the problem in one go. Combine that with the changes above and you break the cycle instead of chasing it.

Written by the team at Tropical Fish Imports in Lethbridge, Alberta. Last reviewed August 2026.

Stuck with an algae problem that will not quit? Bring us a photo and your latest test readings and we will help you find the cause — visit the shop, call 403-380-4506, or send us an email.