Why do some new fish die within 24 Hours?

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In this hobby you'll sometimes get new fish that mysteriously within 24 hours of being added to your tank. There's a couple of factors that can play into this as to why they would pass suddenly, which I will list below.
Acclimation stress - The fish are acclimated too fast, and the stress can put their body into shock. Making sure you acclimate properly to the temperature and water parameters of your tank will help a lot. Also dimming the lights upon introduction and slowly ramping them up over the course of a few hours will help diminish any shock.

Water quality - Make sure before adding in any new fish that you are testing your water to make sure your ammonia, nitrites, nitrates, and salinity are all in check.

Stress from shipment- Sometimes the stress from being shipped overnight can take a toll on their body, especially more sensitive species such as puffers or wrasse. When buying online it's always good to check and make sure the company you buy from has a DOA policy.

What are some ways you try to avoid any sudden losses when adding new fish in?

 

[exnisstech]

TBH I can't remember the last time I lost a fish that quickly. But I'm selective on the fish I buy and do not have fish shipped because I won't buy if I can't see the fish and also verify it isn't being housed in low salinty and or low level copper. I feel that's part of why I'm able to get by running an observation tank vs a medicated QT. I think many early deaths are just unhealthy fish to begin with but that's JMO with no proof, just a few tanks full of years old fish as evidence.

 

[EricR]

This has happened to me twice and my suspicion is that the fish just wasn't strong/healthy for whatever reason.

In each instance (two completely separate events), I bought a pair of the same type of fish (same time / same place / same acclimation) and one of the fish died after a day while the other was fine and lived for years.