Fish
A new tank needs a month before fish move in
Cycling grows the bacteria that convert ammonia to harmless nitrate. Adding fish on day one is the most common cause of them dying in week two.
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Fish recognise the person who feeds them
Archerfish can tell human faces apart, and many aquarium fish gather at the glass for a specific person. They also learn feeding times to the minute.
Goldfish need a pond-sized tank, not a bowl
They grow to over 20 centimetres, live decades and produce heavy waste. Bowls stunt them and shorten their lives dramatically.