how do fish balance the amount of salt and water they take in?

Answer 1

While PeeTee is correct, I suspect you want more information on how this is done. It's all a part of osmoregulation, where the concentration of ions inside the cells is regulated by the amount of water a fish takes in, what the kidneys excrete, and use of active transport to increase the amount of salt inside the cells. Since salt can't move freely across cell membranes, the amount inside the cell is increased or decreased (depending on need) by moving the ions through special channels in the cell membrane that are tied with the removal of other ions (active transport). Note that "salt" doesn't restrict this to NaCl, but any halide compounds, such as potassium chloride, calcium chloride, and others.

Freshwater also have cells (chloride cells) which help in the extraction of salts.

Answer 2

It doesn't matter. The salt and the water are mixed evenly already, so the fish just takes in its surroundings. There is never too much salt or never too much water. The ocean itself balances it evenly, so the fish never have any trouble. =]

Answer 3

Kidneys. Seriously. Look it up.