You would put 1 rounded tablespoon of aquarium salt (table salt has chemicals in that are harmful) per 19l/5 US gallons, it will help reduice the effects of Nitrite and help gill function. It can't be used with catfish or loaches though and you'll need to do a big water change befpre hand because of what you already put in the tank.
I am from Glasgow originally but lived in a place called Helensburgh for 7 years before moving to Newcastle when I was almost 18.