What to do when your new fish dies?

Considering it would csot me alot more in gas than what I paid for a single fish, no, I've nver taken one back.
 
take out the dead fish and freeze it ( the workers appreciate it, we really don't like looking at dead fish at all, much less mushy day old dead fishy bodies) and bring it in with your receipt. Also bring in a water sample so the worker can determine if something caused the death. Don't be afraid about saying if the water was out of wack or cycled wrong or something. I once had a customer come in with a huge ammonia spike and 2 dead fish. generally the worker isn't going to yell at you, the worst i'll do is write up the return and tell you to come back when the tank is more settled (and give back the receipt so no one gets cheated out of the return policy). Most places seem to have a 5-14 day policy on fish, Petsmart has a 14 day.
 
Ask if they have a guarantee at all before even buying the fish. That's the best approach.

Some mom and pop shops do, but a lot don't (that's a tough order to fill when you're a small business).

Every chain store has a guarantee.

My LFS is the best in the SGV. They have great prices, including quantity discounts (i.e buy 4 of the same fish for cheaper price). Out of 10+years of going there, I've never seen a dead fish, so it's understandable that they have no guarantee. There's a sign in the store, and it's on the receipt.
 
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