🧭 Why algae happens (technical lens)

Algae growth = f( light × nutrients × weak competition × poor transport )

• 💡 Light: intensity (PAR), duration (photoperiod), spectrum → sets the energy budget (DLI).
• 🧪 Nutrients: dissolved N (NO₃/NH₄⁺), P (PO₄³⁻), trace metals (esp. Fe), silicate (diatoms).
• 🫧 CO₂ / O₂: in planted tanks, unstable CO₂ = plant stress ⇒ algae gain.
• 🌊 Transport: flow removes boundary layers on leaves/rock, delivering CO₂/nutrients and sweeping detritus; dead spots favor films/mats.

⚖️ Non-zero nutrient targets beat “starvation.” Starving plants/corals weakens your competitors—algae win.

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🪟 Windows & ambient light (silently raises DLI)

• Indoors daylight ≠ dim: Scattered skylight plus wall reflections can deliver meaningful PAR.
• Short sun slivers (10–30 min) raise DLI disproportionately and can trigger spot algae where the beam hits.
• Seasonality: Summer sun path may clear neighboring buildings/trees → midyear blooms.
  Mitigations: Blackout curtain/film on the window, opaque background/side panels, move tank off exterior walls, or angle the tank to avoid direct skylight. If you must stay: reduce fixture photoperiod 30–60 min and add fast growers to absorb the extra energy.

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🔎 Identify the algae (so you pick the right fix)

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🧪 Numeric targets (stable ranges that work)

Freshwater planted (general):

• NO₃ 10–20 mg/L • PO₄ 0.1–0.5 mg/L • K 10–20 mg/L • Fe ~0.05–0.1 mg/L
• CO₂ ~20–30 mg/L consistently from lights-on (use 4 dKH drop-checker solution; aim lime-green).
• Photoperiod 6–8 h (low/med tech). Avoid >8 h unless CO₂ & nutrients are dialed.

FOWLR (no corals):

• NO₃ 5–20 mg/L • PO₄ ≤0.1 mg/L • Photoperiod just for viewing (shorter is fine).

Reef (soft/LPS/SPS):

• NO₃ 5–15 mg/L • PO₄ 0.03–0.08 mg/L (not zero) • In-tank flow 20–30× randomized.
• Photoperiod 8–10 h, stable spectrum, don’t chase daily.

🧪 Test kit caveat: hobby kits vary—trend over time > single readings. Don’t “chase decimals.”

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🛠️ 14-Day Anti-Algae Protocol (planted or reef-safe steps)

Day 1 — Reset & verify

1. Full tests. Confirm NH₃/NO₂ = 0; record NO₃/PO₄ (or NO₃/PO₄/alk for reef).
2. Light: drop photoperiod by 1–2 h; raise fixture or reduce intensity one step.
3. Window control: close curtains, add side panel; if sun hit is unavoidable, reduce photoperiod another 30–60 min.
4. Manual pull: toothbrush-twirl threads, trim affected leaves, siphon detritus.
5. Flow: re-aim returns/pumps to eliminate dead zones; ensure surface ripple.
6. Water change: 40–50% (match temp; dechlor). Clean prefilter sponge.

Days 2–4 — Lower load

• Cut feeding 25–50%; remove leftovers in 2–3 min.
• Run fresh carbon/Poly-Filter 48 h if water is yellow/odors.

Day 5 — Re-balance nutrients

• Planted: dose to targets (above). Often GSA disappears once PO₄ is adequate.
• Reef: if NO₃/PO₄ at 0, raise slowly (dinos/uglies love zeros).

Days 6–10 — Targeted actions

• BBA: off-tank H₂O₂/“liquid carbon” dab on leaves/wood; rinse, return.
• Green water: install UV; size per manufacturer, run slower flow for better dwell; clean quartz sleeve monthly.
• Diatoms: if persistent, add silicate resin; replace when exhausted.

Days 11–14 — Hold steady

• Keep light/flow/settings unchanged; wipe glass; log trends.
• If still worsening, send me your numbers + gear for a fine-tune.

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🌊 Transport physics (why flow fixes so much)

• A thin boundary layer clings to leaves/rock; low flow thickens it, starving plants/corals of CO₂ and nutrients.
• Increasing broad, gentle flow thins the layer, boosting plant/coral uptake and starving algae of easy surfaces to colonize.
• Aim for a circular path so debris drifts to the intake; break up dead corners.

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🧰 Tools & tactics (advanced but safe)

• Lux/PAR checks: A cheap lux meter can reveal window spikes (relative readings AM vs midday). PAR meters are best; don’t convert lux→PAR blindly (spectrum matters).
• UV sterilizer: For water-column algae & some microbes. Effectiveness = wattage × dwell time; slower flow = stronger kill. Prefilter fine debris so UV sees clear water.
• Refugium/chaeto (reef): Competes for N/P; harvest weekly to export nutrients.
• GFO/PO₄ absorbers: Use in short runs; do not bottom out PO₄ (risk of dinos & coral stress).
• CO₂ timing (planted): Start 60–90 min before lights on so plants hit the day with full CO₂; keep pH drop consistent day to day.
• Photoperiod “siesta”: Splitting into two shorter windows can curb mid-day spikes—use cautiously; stability is better.

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🦐 Cleanup crews (helpers, not cures)

Freshwater: Amano shrimp, nerites, true Siamese Algae Eaters, otocinclus, bristlenose plecos.
Marine/Reef: trochus/astrea/turbo snails, conchs (sand), some hermits (with caution), tuxedo urchins.

Always fix cause first; CUC maintain gains.

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🧹 Preventive routine (keep it boring)

• Weekly: 20–40% WC, prefilter rinse, glass wipe, detritus siphon.
• Monthly: service pumps/impellers; ensure heater and leak sensors OK.
• Lighting: timer-controlled; avoid frequent spectrum/intensity fiddling.
• Windows: keep shades down during peak sun; re-assess every season.

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🏳️ Not an end-all, be-all

Every tank has its own “sweet spot.” Use these ranges and methods as a framework, then tune for your light, window exposure, stock, and gear.