#corydoras

I currently have gravel in my aquarium but really enjoy having catfish in my tank especially Corydoras , some people have mentioned I should change my gravel to sand as it’s better for their whiskers and bellies on the bottom, but I have substrate mixed in with the gravel , is there any easy way...

I think my corydoras is showing early signs of swim bladder disease but I’m not 100% would if be harmful to treat the tank even if it isn’t swim bladder disease just as a precaution ?

I cleaned my tank yesterday and did a 25% water change, I noticed last night my Cory had a red blotch on its side and when goggling it suggested to do a salt bath for 15mins and return the Cory back to the tank , done this and this morning the red blotch has gone however still one very unhappy...