So should I quarantine him? I have other things in the tank, will they be affected by the salt?What's the actual problem with the fish?
If you are concerned about coloured stringy poop, the fish needs more plant matter in its diet.
The fish has slightly flared gills, which could be gill flukes. 2 heaped tablespoons of rock salt per 20 litres of tank water should treat that.
The fish hiding is probably from being bullied by the other male.