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Normal stress reaction. Her bars should return after she is fully acclimated and comfortable in the tank. You can speed that up with perfect water quality, plenty of hiding places, dither fish and target fish and while she is new, keep the lights low and increase the lighting up to normal over the period of a few days or so. Also, double check the water temperature, make sure it IS what it should be. I won't try to tell you exactly what temp. it should be because there is a lot of disagreement on what temp's. should be for new fish or even if they should be changed for new vs. normal, etc.
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