FastAquatics
Step-by-step how-to

How to add new fish to an existing tank

Step-by-step protocol for introducing new fish to an established aquarium. Quarantine, acclimation, lights-out introduction, observation period.

Beginner8 steps

What you'll need

Step-by-step

Step 1: Quarantine the new fish first21-30 days saltwater (with prophylactic copper) or 14-21 days freshwater (observation only) in a separate QT tank. Mandatory for marine; highly recommended for freshwater.
Step 2: Drip acclimate to display parameters60-90 minute drip acclimation. Match salinity + temperature within 1°F + 0.001 SG.
Step 3: Plan introduction at lights-offAdd at the END of the lights-off cycle (or with lights off entirely). Resident fish are calmer + less territorial in the dark. New fish acclimates without immediate stress.
Step 4: Rearrange the rockwork (optional)For semi-aggressive species: move the rockwork before adding the new fish. Disrupts existing territories so the new fish has equal claim.
Step 5: Net into the displayNever pour shipping water. Use a fine-mesh net.
Step 6: Lights low for 24 hoursKeep lights at 30% or off for first 24 hours. Don't feed for 24 hours so the new fish acclimates without digestive stress.
Step 7: Observe for 7 daysWatch for aggression - chasing, fin nipping, hiding. If sustained, separate the aggressor and try again with rockwork rearranged.
Step 8: Hold off on additions for 30 daysAfter successful introduction, wait 30 days before adding more fish. Stable bio-load + parameters.

Frequently asked questions

How do I introduce a new fish without aggression?

Add at lights-off, rearrange rockwork to disrupt territories, add multiple peaceful fish at once if needed (dilutes aggression).

Can I skip quarantine for fish from a trusted source?

Marine: never. Even reputable vendors miss carrier-state ich/velvet. Freshwater: lower risk but still recommended.

How many fish can I add at once?

Marine: 1-2 max per addition. Freshwater community: 4-6 (small school species). Always within bio-filter capacity - avoid more than 25% increase in bio-load per addition.

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Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.

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