The cells of the fish will lose water to the aquarium water by osmosis, shrink up, and probably die with time.
1) The tank water is hypertonic to the cells of the fish. Hypertonic solutions are those with higher solute concentrations compared to the sap of the cells placed in them. Hypotonic solutions have lower while isotonic solutions have the same concentrations as the sap of cells placed in them.
2) Because the tank water is hypertonic, the cells of the fish will lose water to the water in the aquarium. When this happens, the cells shrink, and may eventually start dying if the situation remains unchanged enough.
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