Water quality, and even more important diet is what I believe causes Lateral Line.
I received a Purple Tang with a nasty little case of it a few months back. It has been in my reef eating a sheet of Nori per day (increasing to twice per day), as well as Rotifeast, Oysterfeast, frozen Mysis Shrimp - Formula 1 cubes, Large Angelfish food cubes, Dwarf Angelfish food cubes, cyclopeeze cubes, pysis shrimp. All of the frozen foods are thawed rinsed, and then soaked in Selcon, Vitamin C, VitaChem for at least 15 minutes.
I have to apologize for the quality of the pix. I am no good with the cam & reef. The fish has become much faster making it difficult for me to take it's pic.
This 1st. pic is of the right side of the PT when I first got it.
This one is after. Blurry but you can see the HLLE has almost gone.
I will continue to try for a clearer one.
This is the left side before.
Left side after 8 months in my reef. Again I will continue to try for less blurry pix.