Troubleshoot
Too Much Light in a Planted Tank: Algae & Melts
Excess light without matching CO₂, nutrients, and plant mass grows algae—not “better plants.” Hair algae on slow growers and BBA on flow edges are classic. Dial light before you buy another bottle. Related: hair algae removal and green water.
- Too much light
- Hair algae
- Photoperiod
- CO₂ balance
What you’re seeing
8-hour photoperiod on a 100+ PAR bar over a low-tech scape. Algae on Anubias, melt on carpeting stems, drop checker blue while lights blaze.
| Pattern | Likely driver |
|---|---|
| Hair on slow leaves | PAR > CO₂/ferts |
| BBA on filter outflow | High light + unstable CO₂ |
| Green water in parallel | Suspended algae |
| Plants pearl, algae wins | Photoperiod too long |
Likely causes (ranked)
Full-power LED on a “low tech” tank
High PAR without pressurized CO₂ is an algae farm.
Quick test: You run 100% dimmer on a nano with liquid carbon only.
Photoperiod 10+ hours
Even correct PAR melts schedules if the tank rests too little.
Quick test: Timer says 10 hours because “more light = more growth.”
Unstable CO₂ while chasing color
Plants stop; algae on edges where CO₂/flow wobble. See <a href="https://aquaplantshub.com/co2-drop-checker-color-meaning/">drop checker</a>.
Quick test: Checker swings yellow-blue weekly.
Sparse plant mass under strong light
Light hits glass and substrate—algae moves in. Nitrate creep helps: <a href="https://aquaplantshub.com/high-nitrates-planted-aquarium/">high nitrates</a>.
Quick test: Hardscape-heavy, few stems, long photoperiod.
Fix order
-
Step 1
Cut photoperiod to 6–7 hours for two weeks
Or drop intensity 30%. One lever at a time so you know what worked.
-
Step 2
Stabilize CO₂ before adding more light
Green drop checker at midday, solenoid synced. Leaks break balance—leak check.
-
Step 3
Add fast growers or floaters to shade
Plant mass eats light and exports nutrients. Manual remove hair algae you already have.
-
Step 4
Trim, water-change, then slowly raise light
30–40% WC weekly: calculator. Only increase PAR after algae stops winning for 10 days.
When to escalate
- Pea-soup green → green water.
- Hair everywhere → hair algae playbook.
- CO₂ yellow + fish gasp → drop checker + turn gas down.
- Heater cooked plants after light boost → heater check.
Helpful products
Related guides
- Hair algae — Filaments
- CO₂ drop checker — Gas balance
- Green water — Suspended algae
- High nitrates — Nutrients
FAQ
How many hours of light for planted tanks?
Start 6–7 h high-tech, 7–8 h low-tech. Increase only when CO₂ and mass keep up.
Is algae always too much light?
Often light + imbalance. But excess PAR is the usual knob in LED era.
Will lowering light hurt red plants?
Briefly. Stable beats pink melt covered in hair algae.
Can floaters fix too much light?
They help shade—still reduce photoperiod if PAR is extreme.
BBA means too much light?
Usually unstable CO₂/flow at bright zones—not only hours.
Do I need a PAR meter?
Nice, not required. Algae on Anubias under your bar is enough data.
Aqua Plants Hub Editorial
