You probably won't see goldfishes swimming in the ocean, and clown fishes are amusing, but you are not going to see them at a freshwater lake near you. There is a good reason for that, and that is because animals that live in freshwater have a different solute concentration or different concentrations of dissolved matter in their body than animals that live in saltwater. So what would happen if you put saltwater in a goldfish tank?
Water tends to flow from one place to another. Unless it is in an impermeable container, like a water bottle, it is probably moving around. Solute concentration plays a big role in determining where that water flows. The general rule is that water travels from an area with a low solute concentration to an area with a high solute concentration. For now, you just need to remember that water goes from low to high. If you didn't get that, don't worry. Hopefully, this analogy will explain the rule better.
Now, think of a time when you spilled a glass of milk. The milk is all over the table, and you are quickly trying to reach over for a paper towel. Pretend the dryness of the paper towel represents a high solute concentration. The paper towel is very dry right now. The milk is obviously wet and not dry, so it has a very low solute concentration. Using our previous rule, you would expect the milk to get soaked up by the paper towel. Eventually, the paper towel will soak up too much milk and become really soggy. It is now as wet as the milk, so they both have an equal solute concentration. Neither side is losing any liquids, and this is what happens when a freshwater fish is in a freshwater lake.
Fish usually don't soak up the water around them and expand, and they probably aren't releasing too much water and shriveling. That is because the solute concentration inside the fish is the same as the solute concentration of the water. If you add saltwater to a goldfish's tank, you're raising the solute concentration of the water. It now has a higher solute concentration than the goldfish. The goldfish is now like the spilled milk, and the paper towel is all around him. Water is going to leave his body and enter the tank. If the tank becomes too salty, the goldfish will shrivel up and die, so in this case, the grass isn't greener on the other side.