Green film on the glass. Green dust on the sand. You clean it… and it’s back tomorrow. If you’re dealing with green microalgae in a reef tank, you’re not alone—and you don’t need “miracle” chemicals to win. The fastest reef-safe progress comes from correct ID, manual export, and stabilizing nutrients, light, and flow.
What “green microalgae” is (and why it shows up)
Green microalgae is a broad, beginner-friendly label for small algae growth that forms thin layers or dusting before it becomes thicker turf or hair algae.
Common types beginners see
- Green film on glass
- Smooth layer, wipes off easily with a magnet cleaner or scraper.
- Green dust on sand/rocks
- Looks like a light coating; often blows off with a turkey baster.
- Early-stage turf/hair algae
- Starts as “micro” fuzz on rock edges, then can grow into visible strands if left unchecked.
Why it’s common in young reef tanks
In newer systems (especially the first few months), algae often appears during the “ugly phase” because:
- Nutrient pathways are still stabilizing (bacteria and microfauna populations are developing).
- Fresh rock and new sand can trap detritus or leach bound nutrients.
- Lighting and feeding routines are usually still being dialed in.
Safety note: Algae itself isn’t usually the emergency—rapid parameter swings and oxygen drops from aggressive treatments are the real risks.
Identify it correctly before treating
Correct identification keeps you from using the wrong fix (for example, treating cyano like algae, or chasing nutrients when it’s really a flow problem).
Green microalgae vs diatoms vs cyano
| Common culprit | Typical color | Texture | Bubbles? | How it removes | Most common driver |
|---|---|---|---|---|---|
| Green film/dust algae | Bright to dark green | Smooth film or dusty coating | Usually no | Wipes/scrapes off | Light + available nutrients |
| Diatoms | Tan to brown | Powdery, dusty | No | Wipes easily; returns fast in new tanks | Silicates + new tank maturity |
| Cyanobacteria (cyano) | Deep red, maroon, purple, sometimes dark green | Slimy sheet/mat | Often yes (trapped bubbles) | Peels off in sheets | Low flow zones + imbalance (often low NO3 with available PO4) |
Quick checklist (2 minutes)
- Where does it grow first?
- Glass facing the light = often green film algae.
- Sandbed corners/behind rocks = often detritus + low flow.
- How fast does it return after cleaning?
- Same day = usually high light/available nutrients or new-tank instability.
- Does it peel in a sheet and trap bubbles?
- Strongly suggests cyano rather than green microalgae.
When to test and what results matter
Test at the start of your plan, then again after 7–10 days:
- Nitrate (NO3, mg/L or ppm)
- Phosphate (PO4, mg/L or ppm)
- Alkalinity (dKH) stability matters more than chasing a “perfect” number.
- pH trend (especially if you suspect low flow/low oxygen areas)
If corals look irritated, test sooner and avoid big changes until you know where your numbers are.
Root causes (the big three: nutrients, light, flow)
Nutrients: “too much,” “too little,” and imbalance
Green microalgae thrives when it has fuel. That fuel often comes from:
- Overfeeding (especially fine foods that drift into rocks)
- Detritus buildup in sand and rockwork
- Weak export (skimmer underperforming, neglected filter socks/floss, no refugium, infrequent water changes)
But “too clean” can also backfire:
- Ultra-low nutrients (reading 0 mg/L NO3 and 0.00 mg/L PO4) can destabilize a reef and sometimes encourages nuisance microbes (including cyano) because competitors and coral/bacterial balance shift.
Light: intensity, spectrum, and photoperiod
Light doesn’t “cause” algae by itself—it amplifies whatever nutrients are available.
Common triggers:
- Long photoperiods (especially 10–12+ hours at strong intensity)
- Too much white channel on LEDs (often grows film algae fast)
- Old bulbs (spectrum shift) or misconfigured LEDs (too intense too long)
Flow and dead spots
Low-flow areas collect detritus and become algae farms.
Look for:
- Sandbed corners where debris settles
- Behind rock structures
- Areas with low surface agitation (reduced gas exchange)
Safety note: Increasing flow is reef-safe, but don’t blast LPS corals or constantly sandblast tissue. Re-aim gradually and observe coral response.
Beginner-safe action plan (first 7–14 days)
This plan is designed to remove algae you can see while fixing what grows it back.
Step 1 — Manual removal (do this first)
Remove biomass before making changes. Otherwise, algae continues to release spores/fragments and consume nutrients in messy ways.
- Scrape glass
- Use a proper algae scraper; keep blades away from silicone seams.
- Siphon sand
- Lightly vacuum the top layer to export detritus without deep-stirring the whole bed.
- Brush rocks
- Use a soft toothbrush in-tank while siphoning nearby to catch debris.
Export what you remove:
- Run filter floss or a sock during cleaning.
- Change/clean it the same day (or within 24 hours).
Step 2 — Improve mechanical filtration
For the next 1–2 weeks, be more aggressive than usual:
- Replace filter floss every 1–3 days.
- Rinse filter socks every 2–3 days.
- Empty and wipe skimmer cup more frequently so performance stays consistent.
Step 3 — Adjust feeding without starving the tank
Goal: reduce waste, not nutrition.
- Measure food for a week (don’t “free pour”).
- Rinse frozen food in RO/DI water through a fine net to remove packing juices.
- Feed smaller portions more consistently (example: once daily or every other day depending on stocking).
- Remove uneaten food after a few minutes if it’s accumulating.
Step 4 — Tune nutrient export
Pick the tools you already have, then improve consistency.
- Protein skimmer basics
- Keep the air intake clear.
- Aim for a stable foam head; avoid constant overflow.
- A slightly “wetter” skim can help export more during cleanup (watch salinity).
- Refugium/macroalgae
- Chaetomorpha (chaeto) competes for nutrients when lit properly and harvested.
- Targeted water changes
- 10–20% weekly is a common beginner-friendly range during a bloom.
- Match temperature and salinity carefully (salinity: 35 ppt / 1.026 SG is a common reef target).
Safety note: Avoid stacking multiple “export boosters” at once (big water changes + heavy media + major feeding cuts). That’s how tanks swing into instability.
Step 5 — Optimize flow
- Re-aim powerheads to eliminate dead zones.
- Create randomized flow (alternating modes if available).
- Ensure strong surface agitation for gas exchange.
A simple check: after feeding, watch where particles settle. Those “settling lanes” are your next flow targets.
Step 6 — Light adjustments (safe ranges)
Make modest changes and hold them steady for at least a week.
Beginner-safe adjustments:
- Reduce total photoperiod to 8–9 hours of main lighting.
- Reduce white intensity (often the biggest algae accelerator) while keeping blues reasonable for coral viewing.
- Avoid drastic cuts like going from 10 hours to 4 hours overnight unless you’re dealing with an emergency (and even then, be careful).
Target parameter ranges (beginner-friendly)
You don’t beat algae by chasing zeros. You win by keeping corals healthy and limiting excess fuel.
Nitrate and phosphate targets for stability
Practical stability targets many mixed reefs tolerate well:
- Nitrate (NO3): 5–15 mg/L (ppm)
- Phosphate (PO4): 0.03–0.10 mg/L (ppm)
Why “zero” can backfire:
- Corals and beneficial microbes need consistent nutrients.
- Readings of 0 can be a test limitation (below detection) while algae is still consuming nutrients as they enter the system.
Why alkalinity stability matters during algae battles
Algae cleanups often involve changes to feeding, filtration, and water changes. That can unintentionally swing alkalinity.
Beginner-friendly stability goal:
- Alkalinity: ~7–9 dKH, kept steady (avoid changes > ~0.5 dKH/day)
If you dose alkalinity, don’t change your dosing schedule drastically during an algae battle unless tests clearly show drift.
RO/DI water checks
Source water can quietly drive algae problems.
- Aim for RO/DI TDS: 0 ppm (or as close as your meter allows).
- Replace RO/DI filters/resin when TDS rises or when product water quality declines.
- If using mixed saltwater from a store, ask for their RO/DI TDS and maintenance routine.
Clean-up crew and biological helpers (what actually works)
Clean-up crews help maintain surfaces after you manually remove algae. They rarely fix a root-cause bloom alone.
Best beginner options
- Trochus snails
- Strong glass and rock grazers; often can right themselves if they fall.
- Turbo snails
- Powerful algae eaters, but can bulldoze loose frags.
- Cerith snails
- Good for film algae and detritus on rock and sand surfaces.
- Nassarius snails
- Not algae eaters (mostly scavengers), but help stir sand and reduce leftover food buildup.
Tip: Add clean-up crew gradually. Too many animals in a low-food tank can starve once you get algae under control.
Fish options (only if tank size allows)
- Blennies (like lawnmower/starry blennies)
- Can graze algae, but may not eat all types of film algae consistently.
- Tangs
- Effective grazers in appropriate tank sizes, but require space, oxygen, and careful compatibility planning.
Safety note: Don’t buy a fish as a “tool” if your tank volume and long-term care plan don’t fit the species.
Refugium macroalgae as a competitor
A refugium can reduce algae pressure by competing for NO3/PO4.
Chaeto basics:
- Provide steady refugium lighting (often opposite the display schedule helps pH stability).
- Ensure good tumbling flow (not a packed brick).
- Harvest regularly (weekly/biweekly). If you don’t harvest, you don’t export.
What to avoid (common beginner mistakes)
Overcorrecting nutrients
Avoid:
- Huge feeding cuts that crash NO3/PO4 to undetectable.
- Aggressive phosphate removers without testing (PO4 can drop too fast).
- Rapid, repeated big water changes that swing salinity/alkalinity.
“Miracle” algae chemicals
Many quick-fix products can:
- Stress invertebrates
- Disrupt beneficial bacteria
- Lower oxygen (especially if a lot of algae dies at once)
If you ever choose a chemical approach, use it as a last resort, follow directions exactly, increase aeration, and monitor livestock closely.
Blackouts and extreme light cuts
Short blackouts can help some situations, but they often rebound if nutrients/flow aren’t addressed.
Use caution:
- Corals still need stable light routines.
- Blackouts don’t remove nutrients—they only pause growth.
Long-term prevention (a simple weekly routine)
10-minute weekly maintenance checklist
- Clean glass (magnet/scraper)
- Turkey baster rockwork before a water change (suspend detritus)
- Siphon small detritus pockets you can reach
- Empty/wipe skimmer cup and check air intake
- Replace/rinse filter floss or socks
- Test NO3 (mg/L) and PO4 (mg/L) weekly until stable; then space out as confidence grows
Monthly checks
- Clean powerheads and inspect flow pattern
- “Flow audit”: look for new dead spots as coral grows
- Check RO/DI TDS and replace cartridges as needed
- Review light schedule and intensity (avoid creeping longer photoperiods)
When to seek help
Get help if you see:
- Coral tissue recession, persistent closed polyps, or rapid color loss
- Fish gasping at the surface (possible low oxygen)
- Unexplained swings in salinity, alkalinity (dKH), NO3, or PO4
- A bloom that persists >4–6 weeks despite consistent maintenance
Troubleshooting: If it keeps coming back
Hidden nutrient sources
Common “it’s still coming back” culprits:
- Detritus trapped under/behind rockwork
- Dirty sandbed zones (especially if flow is weak)
- Overstocking relative to filtration
- Old or overloaded media (sponges, bio-blocks, floss left too long)
- Skimmer underperformance (air intake clogged, inconsistent water level)
Testing pitfalls
- Expired reagents or improperly stored kits
- Inconsistent sampling time (test at the same time of day if possible)
- Interpreting “0” as true zero (may be below detection while algae consumes nutrients quickly)
A staged escalation plan
Use a stepwise approach so you know what actually helped:
- Increase export consistency (mechanical cleaning, skimmer tune, regular water changes)
- Adjust light gently (shorter photoperiod, reduce whites)
- Upgrade flow (eliminate dead spots, improve surface agitation)
- Consider targeted media (small, measured use of phosphate media with frequent testing)
FAQ
What causes green film algae in a reef tank?
Most often it’s a mix of available nutrients (NO3/PO4) plus strong or long lighting, especially in newer tanks. Detritus trapped in low-flow areas is a very common fuel source.
How do I get rid of green microalgae without harming corals?
Start with manual removal + export (scrape, siphon, floss/sock), then stabilize NO3 (5–15 mg/L) and PO4 (0.03–0.10 mg/L), improve flow, and make small light adjustments (8–9 hour photoperiod). Avoid rapid parameter swings.
Should nitrates and phosphates be zero to stop algae?
No. Chasing 0 mg/L NO3 and 0.00 mg/L PO4 can destabilize a reef and sometimes makes other problems (like cyano) more likely. Stability beats “perfect” numbers.
How long should reef tank lights be on to prevent algae?
Many beginner reefs do well with 8–9 hours of main lighting. If algae is persistent, reduce photoperiod slightly and lower white intensity before making drastic changes.
What’s the difference between green algae and cyanobacteria?
Green microalgae is usually a green film/dust that wipes off. Cyano often forms a slimy mat, commonly red/maroon, and may trap bubbles and peel in sheets—often tied to low flow and nutrient imbalance.
Why does algae come back right after I clean the glass?
Because you removed what you could see, but the system still has enough light and nutrients for fast regrowth. Focus on exporting detritus, keeping filtration clean, and maintaining stable NO3/PO4 rather than cleaning alone.
Call to action: make algae control easier to track
Green microalgae improves fastest when you stay consistent for 2–4 weeks and avoid big swings. If you want help staying organized, log your NO3 (mg/L), PO4 (mg/L), alkalinity (dKH), salinity (ppt), and maintenance steps in Reef Buddy. I’m Shrimpy, and the goal is simple: help you spot patterns (like “algae returns after skipped floss changes” or “photoperiod creep”) so your reef gets cleaner without risky shortcuts.