Why fish gets firm when you salt it?

Fish is just 1 example of how salt may affect food, resulting in Gravlux, i.e Nordic cured fish. 

The reason why salt is making foods more firm is because it starts up the osmosis process. 

When you place a large or small amount of salt on your moist fish, juices present in this fish start to travel from a place of low salt concentration, inside the fish to a place of high salt concentration, outside the fish. This process is know as Osmosis. 

And the fish without its water content is simply a dry, firm fish. 

Et Voila, now you know what ingredient to use or not to use, if you want change up the texture of your fish, cheese or, what the heck, watermelon.