Desalinising/salt controling organs in fish.

Starting with fresh water fish.
From osmosis they leak salts from their body quite quickly, the opposite can be applied to saltwater fish in that salts leak in to them and the fish has to use its kidneys and ion pumps to excrete extra salt and these functions, if salt levels are beyond standard ocean levels, can be stressful to them.

Both fresh and saltwater fish share common organs called an ion pump with in their gill structure, this helps in extracting salt out of the water and is used for greater or lesser needs.

Still with freshwater species.
In order to regain salts lost continually via osmosis, they drink at least four times the amount of water compared to land animals to achieve needed salt levels with in their blood extracted from the low freshwater salt content.

An ion pump abundant with mitochondria cells,is common in both salt and freshwater fish and is also used to discharge ammonias and filter undesirable substances from the water they drink.

Saltwater fish, though not quite as heavy a drinker as freshwater species, still has an internal osmotic concentration lower than that of the surrounding seawater.
So saltwater species tend to lose water and gain salt, so they still drink regularly quite a bit to sustain balanced salt levels by flushing and the fish would literally shrivel up, if not for constant drinking of surrounding waters.

Some marine fish have adopted a different but efficient mechanism to conserve water, this is ia osmoregulation.
They retain urea in their blood in relatively high levels; this helps them to sustain a nearly hypertonic environment with higher solute but lower salt concentration.

This can be damaging to living tissue, to help cope with this problem some fish retain trim ethylamine oxide, this provides a better solution to the potentially toxic urea.
This also means that marine fish urinate very little!

Brackish species.

These fish constantly amaze me with their ability to effectively osmoregulate across a broad range of salinities; fish that can achieve this are known as euryhaline.
Of these types the molly is my favourite.