Aquarium Green Water Phytoplankton Bloom: Causes and Fixes

Suspended single-celled algae can turn an aquarium into pea soup within 48 hours given enough light and dissolved nitrogen. An aquarium green water phytoplankton bloom is not dangerous to fish — fry keepers deliberately culture it — but it ruins visibility and signals an imbalance. This guide from Gensou Aquascaping at 5 Everton Park covers the three reliable fixes, with clear timelines and costs for Singapore hobbyists.

Quick Facts

• Green water is free-floating algae, typically Chlorella or Scenedesmus species
• Triggered by excess nitrate, phosphate, and long or intense lighting
• UV sterilisers at 9 to 18 watts clear tanks under 200 litres in 3 to 5 days
• Four-day total blackout kills the bloom without harming most plants
• Daphnia filter-feed the algae within 48 to 72 hours in small tanks
• Water changes alone rarely solve it — nutrients return from substrate
• Fry breeders intentionally culture green water for first-food infusoria

Identifying Green Water Correctly

True green water is uniform, cloudy, and persists when the water is stirred. If you shine a torch sideways through the tank, the whole beam glows green. Contrast this with green tinted by suspended particulate debris, which settles within 30 minutes in a glass. Free-floating algae remain suspended for days.

Test nitrate and phosphate. Blooms typically occur above 30 ppm nitrate and 1 ppm phosphate, though in newly established tanks with ammonia still converting, a bloom can arrive on much lower nitrate because the cells are using ammonia directly as nitrogen source.

Root Causes Worth Addressing

Direct sunlight on the tank for more than one hour daily is the single biggest trigger in Singapore flats. Even indirect north-facing HDB windows push enough UV through to seed a bloom. Excess light duration beyond 8 hours daily, overfeeding that drives nitrate up, and inadequate water changes form the classic trio.

Pond-style tanks, goldfish setups, and tanks left without weekly maintenance during holiday trips are the most prone. Heavily planted high-tech tanks rarely get green water because fast-growing stems outcompete the phytoplankton for nutrients.

UV Steriliser: The Reliable Method

A 9-watt UV steriliser like the Sunsun JUP-01 at $35 to $45 in Singapore clears a 100-litre tank in 3 to 5 days. Flow rate matters — slower flow gives the UV bulb more contact time. Aim for 4 to 8 times tank volume per hour through the UV unit. Keep the UV running 24/7 during the bloom, then drop to 4 hours daily as maintenance.

Branded options like the JBL ProCristal at $150 offer better build quality but the core algae-killing function is identical. Bulb life is around 8000 hours — replace annually even if the bulb still glows visibly, as UV output drops 30 to 50 percent by end of life.

The Four-Day Blackout

Cover the tank completely with towels or black bin liners, ensuring zero light leak. Turn off the aquarium light. Continue filtration and aeration. After 96 hours, uncover and do a 50 percent water change. The algae cells die from light starvation and settle for the filter to catch.

Most aquatic plants tolerate four days of darkness without visible damage. Sensitive species like Rotala macrandra and Ludwigia pantanal may drop a leaf or two but recover within a week. Anubias, Java fern, and cryptocorynes handle blackouts easily. Do not feed during blackout — waste accumulates and feeds the next bloom.

Biological Control with Daphnia

A handful of live Daphnia magna introduced to green water will filter-feed the algae within 48 to 72 hours in tanks under 60 litres. Source them from breeders on Carousell at $3 to $5 per portion. The catch: any fish in the tank will eat the Daphnia before they finish their job. This method works for fishless tanks or isolated blooms in fry-rearing buckets.

Once the water clears, save some Daphnia for culture in a separate tub. They are an excellent live food for bettas and small tetras.

Nutrient Export to Prevent Return

Once clear, the bloom will return if nutrients stay elevated. Reduce photoperiod to 6 to 7 hours. Keep nitrate under 20 ppm through 30 percent weekly water changes. Add fast-growing stems like Hygrophila polysperma or floating plants like salvinia and frogbit — floating plants are particularly effective because they access both CO2 and nutrients directly at the surface.

Feed lightly. If goldfish are the source of nutrient load, consider rehoming to a pond or upgrading filtration to oversized canisters rated at 8 to 10 times tank turnover.

Green Water in Fry Tanks

Breeders of angelfish, killifish, and livebearer fry intentionally maintain green water as a first food. The phytoplankton seeds infusoria cultures that feed fry too small for brine shrimp nauplii. A 20-litre bucket in a sunny spot with a pinch of fish food develops usable green water within a week. This is the one context where you want the bloom.

Harvest with a fine brine shrimp net or pipette directly into fry tanks at 5 to 10 ml per 10 litres daily for the first two weeks after hatching.

When Not to Panic

A mild green tint in a newly set up tank during the first month often self-corrects as the nitrogen cycle completes and ammonia stops feeding the algae directly. Give it two weeks before intervening. Meanwhile dim the lights and skip the fertiliser dosing. The bloom often clears without action once nitrifying bacteria lock down the ammonia source.

Related Reading

Aquarium Green Water Causes and Fix
Best Aquarium UV Steriliser
Aquarium Blackout Algae Treatment
Aquarium Diatom Bloom Guide
Green Water Aquarium Fix