I think this is called osmoregulation? Basically, the structure of the gills and the gut of fish is so thin that it allows a lot of pass-through between outside world and fishy capillaries, with the difference between blood and water driving the osmosis.

The way they process is split into four main groups:
• doing nothing: there are eel-ish creatures in deep marine layers that simply maintain the same salt concentration as sea water
• recycling: fish like sharks, skates and rays simply pass their urea back into their bloodstream instead of excreting it, effectively raising the salt concentration of their blood up to the level of sea water
• cool kidneys / osmosis: freshwater fish are hyperosmotic, so they're constantly taking in water (see Yishan), but also constantly excreting, for which they have highly efficient kidneys that produce an especially dilute urine; the loss of salts and other solutes is counteracted via osmotic gradient, exchanging salt for ammonia and chloride for carbonate ions
• weird kidneys / drinking water: saltwater fish are the opposite (hypoosmotic), and hence have kidneys developed to extract very little water from the blood (some even lack certain kidney structures and can't eliminate water); this doesn't compensate fully, and the difference is made up in part by "drinking" through the mouth and diffusing through gills, while elimination of solute buildup is done by special chloride cells in the gills with the help of active transport