I think I got it. Things that trigger cell hyperplasia (abnormal increase) or muco-stimulation in fish. First some boring stuff.
Osmoregulation is the control of the levels of water and mineral salts in the blood. This is important because the fish cells are being bathed in tissue fluid which has the correct amount of water. It is a homeostatic mechanism. Homeostasis is the maintenance of constant interior conditions. VERY important that the fish tissue does not lose or gain water by osmosis because the concentrations of water and salt is the same inside and outside the cells.
Then it got more fun. Most freshwater fish (koi), do not have the physiology to deal with salt being added to their water. What!
Their kidneys and other organs have evolved thousands of years to work perfectly in their environment. When a fish is placed in water with salt added, the fluid within the fish still leaves (urinate), then begins trying to get rid of excess salts but can't. The fishes' body evolved to hang on to salts, not get rid of them. This actually causes the fish to dehydrate. The result is stress on the fish.
I thought I had it. Well wait. If the skin on a fish is virtually impermeable with its mucous coating, the hyperplasia of the slime coat would not be in gaurd or protection of osmoregulation, the fish must feel this NaCl as an irritant.
Most all the osmotic entry of water and diffusional loss of salts is happening at the oropharyngeal cavity and the gill epithelium anyway.