What it looks like
Green threads – from short tufts a couple of millimeters long to strands the size of a palm. They grow on leaves, rocks, equipment, often in the most brightly lit corner. The main sign: they can be wound onto a stick or toothbrush, like cotton wool on a skewer.
Why it appears
Thread algae grows where there is more light than plants can use. The reason is usually not the light itself, but that the plants are lacking something and have stalled: fertilizer ran out, CO₂ levels dropped, they were replanted and are acclimating. The light remains the same, but there's no one to absorb it.
Another common reason is excess organic matter: overfeeding, infrequent water changes, a clogged filter.
What to do
- Remove mechanically. Wind it onto a stick and remove it from the water. This is done first: fragments remaining in the water will give rise to new growths.
- Figure out why the plants stalled. If they aren't growing, thread algae cannot be defeated by anything. Check fertilizers and CO₂ supply – in an aquarium without CO₂, see if the light is on for too long for your plant selection.
- Shorten the photoperiod to 7–8 hours while you investigate.
- Increase water changes to twice a week for a month, if you haven't done them recently.
Who eats it
Young, short thread algae are eaten by Siamese algae eaters and Amano shrimp. Mature, long strands are almost untouched by anyone – you'll have to remove them by hand, and only then will fish help control regrowth.
Be careful with Chinese algae eaters: when young, they do clean glass, but with age, they lose interest in algae, grow large, and start attaching themselves to the sides of slow-moving fish. Introducing them as a tool is a bad idea.