❓ Can I add tropical fish to a brand-new tank?

• ✅ Yes, but only if you bring a mature, oxygenated biofilter (seeded media/filter) and stock lightly.
• ❌ No if the filter is brand new and uncycled → you’ll get ammonia/nitrite spikes that burn gills.

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🧪 What “cycling” means (super quick)

• 🧫 NH₃/NH₄⁺ → NO₂⁻ → NO₃⁻: bacteria convert ammonia to nitrite, then nitrate.
• 🧨 Safe to add fish when a 2.0 ppm ammonia dose is processed to 0 NH₃/NH₄⁺ and 0 NO₂⁻ within 24 h (nitrate rises).

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✅ Three safe paths

1) 🚀 Instant cycle (seeded media — same day)

• 🧱 Move 50–100% of established biomedia/sponges (kept wet & oxygenated).
• 💧 Dechlorinate new water (chloramine kills nitrifiers).
• 🌡️ Match temp & pH (±1–2 °F / ±0.3–0.4 pH).
• 🐟 Stock light (25–50% of final bioload).
• 🧪 Daily tests (7 days) → NH₃/NO₂⁻ < 0.1 ppm; water-change if ≥0.25 ppm.

2) 🧪 Fishless cycle (humane & reliable)

• 🌡️ Set 77–82 °F (25–28 °C) + 🫧 strong aeration.
• 🧴 Dose pure ammonia to ~2.0 ppm (+ optional bottled nitrifiers).
• ⏱️ Test daily → when 2.0 ppm → 0/0 in ≤24 h, you’re cycled.
• 💧 Do 50–80% WC to drop NO₃⁻ < 20–30 ppm.
• 🐠 Add fish in stages (25–33% bioload per week).

3) 🆘 Fish-in cycle (last resort)

• 🐟 Very hardy, very light stocking.
• 🧪 Test twice daily; keep NH₃/NO₂⁻ < 0.25 ppm with
  💧 25–50% WCs, 🧴 detoxifier, 🫧 extra aeration, 🍽️ tiny feedings.
• ⏳ Expect 2–6 weeks; no new fish until 7+ days of 0/0.

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🌊 Water type specifics

• 🟢 Freshwater: plants help; fast growers/floaters consume ammonia.
• 🟤 Brackish: match SG; best to seed with brackish-conditioned media.
• 🔵 Marine: prefer fishless with live/dry rock + nitrifiers; monitor SG/alk/temp; avoid fish-in cycles.

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📏 Numbers to aim for

• 🧪 Readiness test: 2.0 ppm → 0 NH₃/NH₄⁺ & 0 NO₂⁻ in ≤24 h.
• 🟢 Fresh daily ops: NH₃/NO₂⁻ = 0, NO₃⁻ < 20–30 ppm (delicate <10).
• 🔵 Marine: NO₃⁻ 2–20 ppm (reef often 5–15), PO₄ 0.03–0.10 ppm.

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🔥 Myths & pitfalls

• ❌ “New water = safe.” → It’s the filter bacteria, not water, that protect fish.
• ❌ Replacing all media → throws away biofilter.
• ❌ Rinsing media under tap → chlorine kills nitrifiers (use tank water).
• ❌ Switching filter off overnight → bacteria suffocate.

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📈 Smart stocking schedule

• 🗓️ Week 0: Prove 2 ppm → 0/0 in 24 h.
• 🗓️ Week 1: Add 25–33% bioload; test daily.
• 🔁 After 7 days of 0/0: add next group.
• 🧯 If NH₃/NO₂⁻ ≥ 0.25 ppm: pause, water-change, stabilize.

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🆕 If you must add fish today (no seed on hand)

• 🧴 Add a true nitrifier product + 🫧 max aeration.
• 🐟 Ultra-light bioload.
• 💧 Daily large WCs + detoxifier; 🍽️ tiny feeds; 🧪 AM/PM testing.
• 🧱 Best fix: get seeded media from a trusted tank/LFS ASAP.

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🧾 Quick checklist (print me)

• 🧪 Liquid tests: NH₃/NH₄⁺, NO₂⁻, NO₃⁻ (and SG for brackish/marine)
• 🧴 Dechlorinator (handles chloramine)
• 🫧 Strong aeration & steady heat
• 🧱 Seeded media or bottled nitrifiers + pure ammonia
• 📊 Readiness proof: 2 ppm → 0/0 in ≤24 h before full stocking