How do I remove green spot algae?

Why this happens

Green spot algae forms hard circular deposits, commonly on bright glass and slow-growing leaves. It can persist where light is strong and plant growth or nutrient delivery is uneven.

• Long or intense lighting increases surface growth.
• Slow-growing Anubias leaves remain exposed for months.
• Plant nutrient imbalance can reduce competition.
• Infrequent glass cleaning allows deposits to harden.

A small amount on old surfaces is normal and may be grazed by some snails. The aim is manageable growth without damaging plants or using unnecessary chemicals.

What to check first

• Is the growth hard and spot-like rather than soft film?
• Which surfaces receive the strongest light?
• Are new plant leaves healthy?
• Has fertiliser or phosphate dosing changed?

Compare the result with the aquarium’s normal baseline, not only a generic target. Record the time, recent maintenance, new livestock, feeding changes, source-water details and any equipment fault. A short timeline often separates the original cause from secondary symptoms.

A practical action plan

1. 1. Scrape appropriate surfacesUse a glass-safe tool and avoid scratching acrylic.
2. 2. Prune permanently covered leavesDo not damage rhizomes or remove all plant mass at once.
3. 3. Review the light periodReduce unnecessary duration gradually.
4. 4. Keep nutrients stableUse a consistent plant plan and test before making targeted changes.

How to interpret the response

Longer intervals between cleaning and cleaner new leaves indicate progress. Existing hard spots may not disappear without removal.

Change one important variable at a time where welfare allows, then observe and retest. When several products or adjustments are made together, it becomes difficult to know what helped, what caused stress and whether the underlying problem remains.

What to avoid

Do not use metal blades on acrylic, overdose phosphate because of one theory, or expose livestock to concentrated spot treatments.

When it is urgent

Chemical exposure, damaged acrylic, fish gasping or widespread plant injury after treatment requires immediate water changes and reassessment.

How to prevent it next time

Clean glass routinely, shade slow-growing plants, maintain balanced fertilisation and avoid abrupt light increases.

Keep enough records to recognise the aquarium’s normal behaviour, water chemistry and equipment performance. Trends are more useful than isolated numbers, and early correction is usually safer than a large emergency change.