Algae: green slime in the reservoir
Green growth on the water surface, the pebbles or the inside of the bucket. Ugly, usually harmless, but it competes with the roots for oxygen.
What you see
A green film or slimy coating on any surface that receives light and moisture — the top of the pebbles, the inside walls of the reservoir, the water surface itself. In bad cases it forms a mat.
The plant may show no symptoms at all, or may yellow slowly as the algae consumes oxygen and nutrients intended for the roots.
Why it happens
Light reaching the water
Algae need light, water and nutrients — and the reservoir has the last two in abundance. Any gap where light hits the water will grow algae. This is the entire cause in most cases.
An uncovered net pot or lid gap
The commonest source. A lid that does not sit flush, an oversized hole, or an open gap around the net pot.
Nutrients in the water
Obviously present. Algae competes directly with the plant for them, which is why a heavy bloom can slow growth.
Warm water
Algae grow faster in warm water, as does everything else.
What to do
Block the light. That is the fix, and everything else is secondary. Cover the water surface completely — the lid seated properly, blackout tape around any gap, and a cover over the net pot rim.
- Clean the reservoir at the next change. Wipe the walls and rinse the pebbles.
- Check the lid fit. If the hole is larger than the net pot, tape the gap.
- Keep the reservoir cool. Slower growth, less competition for oxygen.
- Do not use algaecides. Anything that kills algae will stress the plant.
Algae on the pebbles above the water line — how are people sealing the lid?
Green film keeps coming back. Lid sits flat, so what else?
All problems
Six things go wrong in a small reservoir. The index covers each.