Maintaing a water balance

Freshwater bony fish constantly gain water by osmosis from the surrounding hypotonic water.
Their kidneys possess many large glomeruli which allow a high filtration rate of blood resulting in the production of a high volume of dilute urine.
Their chloride secretory cells actively absorb salts from the surrounding water.

Salt water bony fish constantly lose water by osmosis to the surrounding hypertonic water. They also gain salts by diffusion across the gills.
They drink sea water and have kidneys with few small glomeruli which produce a small volume of concentrated urine.
Their chloride secretory cells actively secrete excess salts out into the sea.

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