The maintenance of a constant ratio of ions within internal fluids. This ratio will invariably be different than that in the external environment.
All fishes regulate their ionic content.
Only the hagfishes fail to osmoregulate. Highly toxic, it cannot be permitted to build up in body fluids. Involves some energy loss since one carbon atom is lost with each two nitrogen atoms eliminated. All show up in fish urine. Since they are more complex molecules, they involve more loss of chemical energy.