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SESK: Stuff Everyone Should Know · sesk.info/fish-add-one-or-two-at-a-time

Stocking a brand new aquarium: how does Merck say it can be good to add fish?

Slowly, 1 or 2 at a time over several weeks.

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  1. All of them on the first day
  2. A new fish every single day
  3. 1 or 2 at a time over several weeks ✔

Why: Each new fish adds waste. Adding 1 or 2 at a time over weeks gives the filter bacteria time to grow and keep up. Merck warns each addition can bring in disease unless the new fish were quarantined.

Huh, didn't know that: Merck says another option is to stock the new tank and watch ammonia and nitrite closely.

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What do you call a fish with a good job? A reel professional.
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References

Answer: 1 or 2 at a time over several weeks. Each new fish adds waste. Adding 1 or 2 at a time over weeks gives the filter bacteria time to grow and keep up. Merck warns each addition can bring in disease unless the new fish were quarantined.

  1. Merck Veterinary Manual: Providing a Home for Fish www.merckvetmanual.com/all-other-pets/fish/providing-a-home-for-fish
    it can be good to add fish slowly to a new aquarium, 1 or 2 at a time over a period of several weeks.
    Checked 2026-10-11.

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