Green water turns the tank into pea soup from free-floating algae. It looks alarming but is usually easy to clear.
How to recognise it
The water itself turns green and cloudy; you cannot see through it, though surfaces stay clear.
What causes it
A bloom of single-celled algae fed by excess light and dissolved nutrients (overfeeding, too much direct sun).
Heads up
Green water starves plants of light and can crash oxygen at night — clear it, don't ignore it.
How to treat it
- Cut light: reduce photoperiod and block direct sunlight on the tank.
- Run a UV steriliser or fine filter floss to physically remove the algae.
- Reduce nutrients: ease feeding and do water changes to lower dissolved organics.
Prevention
Avoid overlighting and overfeeding, and keep a balanced plant load to outcompete algae.
For background, see the nitrogen cycle and our fish health hub.
What causes Green Water Algae: The Pea-Soup Bloom
Green Water Algae: The Pea-Soup Bloom rarely appears out of nowhere. It is usually triggered by poor water quality, sudden parameter swings, overcrowding, or stress from incompatible tank mates. Understanding the trigger is half the cure, because treating the fish while ignoring the environment only leads to relapse.
Step-by-step treatment
Start by confirming the diagnosis against the signs above, then move the affected fish to a hospital or quarantine tank if practical. Improve water quality first with a partial water change and a check of ammonia, nitrite and nitrate. Follow the recommended medication exactly — dose to the tank volume, complete the full course, and remove activated carbon during treatment. Raise temperature slightly only if the specific treatment calls for it, and monitor closely for the full course.
When to act fast
If a fish is gasping, spinning, or covered in rapidly spreading lesions, treat within 24 hours — delay is the most common reason treatments fail. Isolation reduces stress on the sick fish and protects the rest of the community.
Preventing recurrence
Prevention beats treatment every time. Quarantine all new arrivals, keep the tank cycled, feed a balanced diet, and avoid overstocking. A stable, low-stress tank is the real long-term cure.
What causes Green Water Algae: The Pea-Soup Bloom
Green Water Algae: The Pea-Soup Bloom rarely appears out of nowhere. It is usually triggered by poor water quality, sudden parameter swings, overcrowding, or stress from incompatible tank mates. Understanding the trigger is half the cure, because treating the fish while ignoring the environment only leads to relapse. Good news: the same habits that prevent most diseases — stable, cycled water and low stocking — also make treatment far more likely to succeed.
Step-by-step treatment
Start by confirming the diagnosis against the signs above, then move the affected fish to a hospital or quarantine tank if practical. Improve water quality first with a partial water change and a check of ammonia, nitrite and nitrate. Follow the recommended medication exactly — dose to the tank volume, complete the full course, and remove activated carbon during treatment. Raise the temperature slightly only if the specific treatment calls for it, and monitor closely throughout. Never mix multiple medications unless a trusted source confirms they are compatible.
When to act fast
If a fish is gasping, spinning, or covered in rapidly spreading lesions, treat within 24 hours — delay is the most common reason treatments fail. Isolation reduces stress on the sick fish and protects the rest of the community from contagion. Keep a basic quarantine setup and a small medication kit on hand so you are never shopping for cures while a fish is declining. Early, decisive action saves more fish than any single product.
Preventing recurrence
Prevention beats treatment every time. Quarantine all new arrivals, keep the tank cycled, feed a balanced diet, and avoid overstocking. A stable, low-stress tank is the real long-term cure. After any outbreak, disinfect nets and equipment, and consider a full course of preventive treatment in the quarantine tank for any fish that shared water with the sick one.
