Green Water, Phytoplankton Bloom in the Water Column

Green water is a phytoplankton bloom, single-celled algae have multiplied
to the point where they color the water itself. You can see it by looking
through the tank from the side: the water has a green or yellow-green tint,
like looking through a piece of sea glass. The rock and livestock are visible
but the water isn’t crystal clear.

Phytoplankton are photosynthetic and require two inputs to bloom: nutrients
(nitrate and phosphate) and light. When both are
present in excess of what the tank’s export system can manage, the free-floating
algae population explodes.

What Causes Green Water

• Elevated nitrate and phosphate, the primary drivers.
  Test both immediately if the water is green. Nitrate above 10 ppm and
  phosphate above 0.05 ppm together create strong conditions for a
  phytoplankton bloom.
• Photoperiod too long, 14+ hour lighting schedules give
  phytoplankton more energy than the tank’s competing biology can suppress.
  Even moderate nutrients will produce green water with a long enough photoperiod.
• Direct or indirect sunlight reaching the tank, a window
  nearby adds uncontrolled light hours that the tank’s timer doesn’t account
  for. This is one of the most overlooked causes of persistent green water.
• Overfeeding, excess food breaks down into nitrate and
  phosphate. A tank fed heavily without adequate export will build nutrients
  steadily until a bloom develops.
• Insufficient nutrient export, old filter floss, a
  skimmer running wet or clogged, or skipped water changes all allow
  nutrients to accumulate past the bloom threshold.
• New tank maturation, elevated nutrients during cycling
  combined with lighting can produce a green water phase in weeks 4–10 of
  a new tank. Usually self-resolves as the biological system matures.

How to Confirm It’s a Phytoplankton Bloom

• Fill a clear glass with tank water and hold it up to a white background, a genuine phytoplankton bloom produces a visible green or yellow-green tint in the glass
• Test nitrate and phosphate, both will typically be elevated
• The cloudiness is uniform throughout the water column, not concentrated near the sand or a specific area
• Developed gradually over days to weeks, not suddenly overnight

How to Fix Green Water, Step by Step

1. Test nitrate and phosphate first. Get exact numbers.
   You need to know your baseline before you can measure improvement.
   Target: nitrate under 10 ppm, phosphate under 0.05 ppm.
2. Reduce feeding immediately. Feed only what fish consume
   in 2 minutes, 2–3 times per day. Skip one day per week. Thaw frozen food
   in a cup and strain the liquid before adding, the liquid is high in
   phosphate. This is the most impactful single change you can make.
3. Reduce photoperiod to 8–10 hours. Cut light hours while
   the bloom is active. Less light = less energy for phytoplankton = slower
   reproduction rate. This slows the bloom while you address nutrients.
4. Block any indirect sunlight reaching the tank. Add
   blinds, curtains, or opaque backing to any window the tank faces or
   that allows sunlight into the room during the day.
5. Replace filter floss immediately and then every 5 days.
   Old floss decomposes and releases nutrients back into the water, it’s
   adding to the problem rather than solving it.
6. Check and service the protein skimmer. Clean the neck,
   empty the cup, verify operating water depth is correct. A properly
   functioning skimmer removes dissolved organics that feed the bloom.
7. Increase water change frequency to 10–15% twice per week
   until nitrate and phosphate are back in range. Then return to bi-weekly.
8. Run activated carbon, helps export dissolved organics
   and improves water clarity. Replace monthly while bloom is active.
9. UV sterilizer if available, running a UV sterilizer
   on a recirculating line kills free-floating phytoplankton cells directly.
   This is the fastest mechanical resolution but doesn’t address the
   underlying nutrient cause, combine with the steps above.

Timeline

Green water caused by elevated nutrients clears within 1–3 weeks when
feeding is reduced, photoperiod shortened, and water changes increased.
If it persists beyond 3 weeks despite these measures, the nutrient source
hasn’t been fully identified, recheck for tap water use, a dead animal,
or a skimmer that isn’t functioning correctly.

Green Water in a New Tank

A phytoplankton bloom in the first 6–10 weeks of a new tank is a maturation
phase, the biological competition that would normally suppress it hasn’t
established yet. The intervention is the same (reduce nutrients, shorten
photoperiod) but the expectations are different: it clears more slowly because
the competing biology is still developing. Don’t add livestock to resolve it, patience and nutrient management are the right tools.

Green Hair Algae on Rock, Attached Filamentous Algae

Green hair algae (GHA) is bright green filaments growing directly attached
to rockwork, frag plugs, and glass, not floating free in the water column.
It grows in strands from a few millimeters to several centimeters long.
In heavy outbreaks it covers the entire aquascape in a green carpet that
overgrows coral bases and makes the tank look untended.

GHA is one of the clearest indicators of elevated nutrients in an established
tank. It doesn’t grow in low-nutrient environments. Its presence is a direct
signal that nitrate and/or phosphate are above what the export system can
maintain.

What Causes It

• Elevated nitrate, above 5–10 ppm in a reef tank provides reliable fuel for GHA
• Elevated phosphate, above 0.05 ppm; often a bigger driver than nitrate alone
• Overfeeding, the most controllable nutrient input in most beginner tanks
• Old filter floss, decomposing floss releases phosphate and nitrate directly into the water
• Tap water, adds nitrate and phosphate with every top-off and water change, continuously replenishing what GHA feeds on
• Insufficient skimmer performance, skimmer not pulling skimmate means dissolved organics accumulate unchecked
• Photoperiod too long, fuels growth rate even when nutrients are borderline

How to Fix GHA, The Right Order of Operations

1. Test nitrate and phosphate. Don’t start removing algae
   manually before you know where your nutrients are. Manual removal without
   addressing nutrients produces regrowth within days.
2. Address every nutrient source simultaneously:
   • Reduce feeding, small amounts 2–3x daily, nothing left uneaten
   • Replace filter floss now and every 5 days going forward
   • Switch to RODI water if using tap
   • Service the skimmer, clean neck, verify output settings and water depth
3. Shorten photoperiod to 10 hours while GHA is active.
4. Increase water change frequency, 10–15% twice per week
   until phosphate and nitrate trend down. Test weekly to confirm progress.
5. Manual removal: Once nutrients are trending down, remove
   GHA physically by hand or with a soft toothbrush. Pull it off in clumps
   and siphon it out during the water change, don’t leave removed algae
   floating in the tank to decompose and recycle its nutrients.
6. Add GHA-eating clean-up crew:
   • Turbo snails, effective glass and rock grazers
   • Sea hare, the most aggressive GHA consumer; requires return to fish store or trade once algae clears as they starve without sufficient food
   • Lawnmower blenny, grazes continuously; needs enough algae to sustain it long-term
   • Emerald crabs, eat GHA and bubble algae; occasionally nippy with coral

Why GHA Comes Back

If GHA returns within a week of manual removal, nutrients haven’t been
adequately addressed. The regrowth rate is proportional to the nutrient
excess, very fast regrowth means significant excess, slow regrowth means
you’re close to the balance point. Keep tracking nitrate and phosphate
weekly until both hold at target levels for 3–4 consecutive weeks.

For the full GHA deep-dive:
Common Reef Tank Algae Problems

Green Film on Glass, Microalgae on the Viewing Surface

A thin green or brown-green film on the glass is microalgae, single-celled
algae colonizing the smooth glass surface. It’s the most cosmetically
bothersome form of tank greenness and the least biologically significant.
Every reef tank develops glass film, the question is how fast it grows
and how much surface area it covers.

What Drives Heavy Glass Film Growth

• Elevated nutrients, faster growth rate with more food available
• Photoperiod too long, more light hours accelerate glass film growth
• Low flow near the glass, algae establishes more easily on still surfaces
• Infrequent cleaning, once established, film grows faster from the existing colony

How to Manage Glass Film

• Weekly glass cleaning as part of the maintenance routine.
  A magnetic glass cleaner used once or twice per week prevents film from
  establishing to the point where it significantly affects visibility.
  Clean the front glass on a consistent schedule rather than waiting until
  it’s noticeably green.
• Turbo snails on the glass. Turbo snails graze glass
  surfaces continuously and reach corners and edges that magnetic cleaners
  miss. 2–3 turbo snails in a 25-gallon tank make a meaningful difference
  to glass clarity between manual cleanings.
• For back and side glass: Let it grow. Coralline algae
  on the back glass is desirable and provides natural background color.
  Scraping the back glass disrupts the coralline and microalgae balance
  that provides a natural look, focus cleaning efforts on the front
  viewing glass only.
• For stubborn spot algae that a magnetic cleaner won’t
  remove: a razor blade (for glass tanks, not acrylic) removes calcified
  spots with a single careful scrape. Hold the blade at a shallow angle
  to avoid scratching.

Glass film is a maintenance task, not a system problem. If it’s growing
so fast that the front glass is noticeably green within 2–3 days of
cleaning, that’s a nutrient signal, address nitrate and phosphate,
not just the glass.

Turf Algae on Rock, Dense Matted Green Coating

Turf algae looks like a rough, carpet-like coating on rockwork, shorter
and denser than GHA, with a matted rather than individual-strand appearance.
It ranges from bright green to grey-green to brown depending on species and
light levels. Unlike GHA which can be pulled off in clumps, turf algae is
anchored into the rock surface by holdfasts and requires scrubbing to remove.

What Causes It

Turf algae shares the same nutrient drivers as GHA, elevated nitrate and
phosphate, but it’s more physically established. It typically follows a GHA
phase as the dominant algae shifts to a slower-growing, more resilient form.
Tanks that have had GHA for months without adequate treatment often develop
turf algae as the algae community matures.

How to Remove It

• Mechanical scrubbing, use a stiff toothbrush or algae
  scraper to scrub affected rock sections during water changes. Scrub in
  the tank water, not in air. Immediately siphon the dislodged material
  out before it settles elsewhere.
• Sea hare (Dolabella auricularia), the most effective
  biological control for turf algae. Consumes it rapidly but requires
  rehoming once the algae clears as it will starve without sufficient food.
• Tuxedo or long-spine urchins, graze rock surfaces
  continuously and are effective long-term controls. They rearrange
  rockwork as a trade-off.
• Address nutrients first, turf algae removed without
  reducing nutrients regrows from the remaining holdfast system. Both
  mechanical removal and nutrient reduction are required.

Bubble Algae, Round Green Bubbles on Rock

Bubble algae (Valonia) appears as bright, shiny green spheres attached to
rockwork and frag plugs, ranging from 2mm to 20mm in diameter. It doesn’t
discolor the water or create a film, it grows as distinct round structures
that are visually unmistakable once you know what they are.

The Critical Removal Rule

Each bubble is a single large cell containing reproductive spores. A bubble
that’s popped or crushed inside the tank releases those spores into the water
and spreads the outbreak significantly. Always remove bubbles intact, peel
them off the rock at their attachment point without puncturing them, place
directly into a bag or container, and remove from the tank. Never crush them
in the water column.

Biological Control

Emerald crabs (Mithraculus sculptus) are the most effective biological
control, they consume bubble algae whole without releasing spores.
1–2 emerald crabs per 20 gallons is a reasonable starting count.
Most are reef-safe; occasionally an individual develops a coral-nipping
habit, which is uncommon but worth monitoring.

For the complete bubble algae removal guide:
Common Reef Tank Algae Problems