Pictures of fish and tank?
What symptoms do the fish have before they die?
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If they are dying within a few hours of going into your tank, it is probably shock from a sudden change in water chemistry (pH, GH & KH). Find out what the shop's pH and GH are and find out your own. If there is a major difference that could be the cause.
The GH (general hardness), KH (carbonate hardness) and pH of your water supply can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
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Find out when the shop gets their fish in and buy them a week later so the fish have a week to recover from the move to the shop.
Find out when the shop does water changes and try to buy the fish a day or two before the water change. This means they have a few days to recover from the water change before they go into your tank with potentially different water.
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The following link is about fish importing and shows how much stress aquarium fish go through when they leave a fish farm and end up in a pet shop.
If any of you have wondered how aquarium fish get to the local pet shop, the following will give you some insight as to what they have to go through to get from the fish farm where they are bred and raised, to the pet shop where you can buy them. The stress they go through is quite...
www.fishforums.net