Aquarium Algae Control: Causes and Cures

What it looks like: dusty brown coating on glass and rocks in the first 2–8 weeks.

Cause: silicate from tap water + a brand-new tank with no competition.

Fix: nothing. Diatoms vanish on their own as the tank matures. Wipe the glass weekly and use an RO unit if it lingers past 8 weeks.

Cause: excess nitrate (>20 ppm) and/or phosphate (>0.1 ppm), often paired with too-long lighting.

Fix: cut lighting to 6 hours a day, do back-to-back 20% water changes, manually pull out as much as you can, and reduce feeding by 30%.

Add cleanup crew: turbo snails (marine), Amano shrimp or otocinclus (freshwater).

What it looks like: slimy red, purple or dark green sheet that lifts off in one piece. Often smells musty.

Cause: low flow, high nutrients, decaying matter trapped in substrate.

Fix: siphon it out, increase flow with an extra powerhead, do a 25% water change, clean the substrate. ChemiClean (marine) or EM Erythromycin (freshwater) work as a last resort.

What it looks like: water tinted green so you can't see the back of the tank.

Cause: free-floating algae fed by direct sunlight or excess nutrients.

Fix: blackout the tank for 3 days, do a 40% water change, add a UV steriliser. Move the tank out of direct sunlight permanently.

Looks like green bubbles. Don't pop them in the tank — they release spores. Remove the rock, pop them in a bucket, then return.

Emerald crabs eat it but they need to be hungry. Don't feed them.

All algae comes back unless you fix the root cause: lighting too long, feeding too much, water changes too infrequent. Get those three right and most tanks stay clean indefinitely.