Learn guide
The Light–CO₂–Fertilizer Triangle (Stop Chasing Algae)
Learn guide
Plants need light, carbon, and nutrients in proportion. Raise one corner of the triangle without the others and algae takes the leftover energy. Choose a low-tech or high-tech lane and stay there.
- Balance first
- Pick a lane
- Timer + drop checker
- Algae = mismatch
Quick facts
| Low-tech lane | Low–medium light · no injected CO₂ · light liquid fert 1–2×/week |
|---|---|
| High-tech lane | Medium–high light · stable injected CO₂ · regular macros+micros |
| Photoperiod | Start 6–7 h on a timer; never 12 h “because plants” |
| CO₂ proxy | Drop checker lime-green at lights-on + 1–2 h (high-tech) |
| Light sizing | Use the LED power calculator before upgrading bars |
| Algae signal | BBA ≈ too much light/time vs CO₂; GSA ≈ too much light vs phosphate |
Overview
The triangle is simple: light drives demand, CO₂ and fertilizer supply that demand. A Budget 150 low-tech scape with Anubias and Java fern can look perfect with modest light and a weekly micro dose. The same light on Monte Carlo without CO₂ often melts or grows algae instead of a carpet — see the low-tech vs high-tech comparison.
High-tech means you commit to a regulator, diffuser, and drop checker — start with the CO₂ system guide and Budget 200 CO₂ BOM. Unstable CO₂ plus strong light is a classic BBA trigger. Green spot on slow leaves often tracks GSA under excess PAR.
Size the bar with the LED calculator. Then lock a timer. Fertilizer (e.g. Tropica Premium) comes last — dose to the lane, not to Instagram growth rates.
How to stay inside one lane
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Step 1
Pick low-tech or high-tech before buying gear
Low-tech: epiphytes, crypts, moss. High-tech: carpets, fast stems, red plants. Do not buy a WRGB “for later” and run it at 100% now.
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Step 2
Fix photoperiod with a timer
6–7 hours beats guessing. If algae appears, cut 1 hour before raising fertilizer.
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Step 3
Only add CO₂ if light actually needs it
If you are still on Anubias/Java fern, skip injection. If you want Monte Carlo as a real carpet, inject and verify with a drop checker.
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Step 4
Dose fertilizer to match light + CO₂
Low-tech: 1–2×/week lean. High-tech: follow a simple all-in-one or EI-lite schedule. Never dump macros into a low-light tank “to be safe.”
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Step 5
Treat algae as a triangle error, not a pesticide job
Identify BBA vs GSA vs GDA, then change light hours or CO₂ stability first. Spot-treat only after the imbalance is fixed.
Common problems
BBA on slow leaves after a new LED
Why: Light/time jumped; carbon did not.
Fix: Dim or shorten hours; delay CO₂ until you can run it stably; BBA guide.
Drop checker yellow or fish gasping
Why: CO₂ too high or poor surface gas exchange.
Fix: Increase surface agitation; lower bubble rate; never turn CO₂ on 24/7 without a solenoid/timer.
GSA dots on Anubias
Why: High PAR + low phosphate relative to light.
Fix: Reduce intensity; modest PO₄ if you dose; GSA troubleshoot.
Stems pearling but carpet melting
Why: CO₂ gradient — strong at the top, dead at the substrate.
Fix: Aim diffuser/outflow across the carpet; check drop checker placement.
Gear that keeps the triangle honest
- Timer — non-negotiable on both lanes.
- Drop checker — high-tech only; lime-green target.
- Lean fert — Tropica-style for low-tech; increase only with light/CO₂.
- Next reading: beginner mistakes and CO₂ hub.
Gear for this species
Budget 200 CO₂ setup — If you choose the high-tech lane — regulator + diffuser sized honestly
FAQ
What is the planted tank triangle?
Light, CO₂, and fertilizer must stay in proportion. Raising only light usually grows algae.
Can I run high light without CO₂?
Not for long on demanding plants. Use low–medium light or add stable injection.
Do low-tech tanks need fertilizer?
Often a light weekly micro dose is enough. Soil and fish waste cover some demand.
How do I know CO₂ is in range?
A drop checker that stays lime-green during lights-on, plus fish not gasping, is the practical check.
Should I increase fertilizer to fight algae?
Usually no. Cut light hours first. Extra fert in a high-light, no-CO₂ tank feeds algae.
Is a drop checker required for low-tech?
No. It is for injected CO₂. Low-tech relies on a timer and modest light instead.
Aqua Plants Hub Editorial
