I'm interested in understanding this process (Osmoregulation) better. There is some debate about treating freshwater fish by adding medication to the water, and for some instances adding epsom salt. I have read that osmoregulation in fish only allows water to pass through the skin, and not minerals. http://www.simplydiscus.com/library/biology/anatomy/osmosis.shtml Other articles mention how increased magnesium in the water helps the fish draw in more water thus flushing out the system. http://www.aquarium-pond-answers.com/2006/12/how-do-fish-drink.html#freshwater http://www.americanaquariumproducts.com/AquariumKH.html#magnesium I'm a bit confused as hardness of the water is said to regulate the amount of water a fish absorbs through osmosis, as water moves from an area lower mineral content to an area of higher mineral content. Adding magnesium sulfate to the water would raise GH so how does this relate to increasing the fishes intake of water? So when using medications in the water, are these medications absorbed by the fish through osmosis, and how can epsom salt aid in flushing parasites from the intestines when it is added to the water.