Was the tank cycled for at least a month before adding the fish? What are your water parameters? Even in a cycled aquarium, 8 fish is a lot to throw in at once. My guess, without knowing more information, is that your aquarium is experiencing diminishing water quality and the guppy you are having problems with simply cannot cope.
I would recommend testing your water right away. If water quality is not where it should be, doing a large water change and keeping an eye on the quality would be your best bet without returning any fish.
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