Slime coats and you...yes, they matter.
Fish have an insulation, a slime/moisture that coats their bodies and protects them from both iffy water acidity and chance encounters with parasites; it also may limit bleeding in injury and promote healing of minor scrapes.

Things that diminish the slime coat: the alkalinity of your water being out of bounds (the range should be 7.9-9.0 on the dkh scale, and if you aren't testing your alkalinity weekly, you should be. Daily, when just...