Fish gills regulate ions diftrrenWY in freshwater and seawater 438 BI0834 Animal Physiology Freshwater fish vs seawater fish ion regulation" 2257 views Published on Nov 25. 2014 Fish gills regulate ion differently in freshwater and saltwater Marine water fish blood has a higher water concentration than their surrounding sea water causing water molecules to diffuse out of the water by osmosis blood into their surrounding These fish must replace the lost water by the concentrated water drinking their surrounding intaking salts which is then removed by active transport through their gills Na+-Cl- ATPase molecules, back to the seawater lon exchange in saltwater fish co-transporter brings The Na+ K+/2C- (NKCC) in potassium ion (K+) and chloride ion (Cl-) from the extracellular fluid in the fish gill epithelial gradient electrochemical cells using the Na+ The K+ and the Nat separate from the 2C-in