Osmosis is the movement of water molecules across a semi-permeable membrane from an area of high water concentration to low concentration in order to balance solute levels. Isotonic means solute levels inside and outside the cell are equal, while hypertonic means outside solute is higher causing the cell to shrivel, and hypotonic means inside solute is higher causing the cell to burst. Freshwater fish absorb water through their bodies due to higher internal salt concentration and expel excess through gills and waste, while saltwater fish drink water due to their higher internal water concentration and expel excess through gills and waste.