Why can't you put a freshwater fish in a salt water tank?

Saltwater fish are adapted to cope in a high solute solution. Normally, the high concentration around them makes the water potential low, so water should move out of the cells of the fish into the water (e.g. as it passes through the gills). However, those cells would be adapted to cope and have a high concentration too and there for a low water potential - the water wouldn't move between the cells and the water as there is no water potential gradient.

When the fish is put into fresh water, suddenly the concentration around them is very low and the water potential is extremely high. Water will now move from the relatively high water potential in the water to the relatively low water potential in the cells, the cells start to burst and the fish dies.

The movement of the water is by osmosis.

Hope it helps!

thank you so much, it helpful.

NP glad to help