Red Rainbowfish Poorly

[sillysausage]

I have 2 red rainbowfish and noticed a white patch on one of their mouths. the other one is always chasing it so it it stress causeing this? Its not bad but worried it might get worse. I am told to add salt and it is a 40 gallon tank. Any suggestions to confirm this?

 

[alihar]

Is the patch a fluffy cotton wool like patch or a smooth White patch?
If it's smooth it could just be scar tissue from bumping into things from being chased. Scar tissue doesn't really need treating and will fade over time with good aquarium maintenance.
If it's not cotton wool like and relatively small I would just keep an eye on it for now and see if there is any change.

Alison