Here I go again, and I don't get a commision.
Use methylene blue.
Catch the fish, keep it in the net, use an eyedropper and apply a drop directly onto the affect area.
Don't do this over the carpet, do it over the tank.
release fish back into the tank after holding it in the net for a minute.
It seems a long time, but they will last longer than that on the floor and they are being cradled in a wet net.
Treating like this, you don't have the staining in the tank.
But make sure you don't get any into the gills of the fish
As far as the 1tspn per 20 litres of salt goes.
mmmmm
Leave the unopened bag of salt beside the tank, touching it of course, and IMHO, that's about what that dosage is worth.
We commonly use one tspn per four litres with our killies.
Alan 104