Diseases of the Skin
Holes in the Skin – Are bacterial infections, and may be handled according to this document, wherein you might endeavor to treat the holes in the skin with iodine and woven gauze, ensure a perfect environment and then find a way to get an antimicrobial to work in the fish.
Red Streaks on the Skin – Check your PH and Nitrate with the following test kit. The pH could be very low, or the Nitrates could be very high. It may also be a bacterial issue but I doubt it. There’s a lot of information about how to treat bacterial infections but consider THIS.
Blackening of the Skin – That’s a color change and you should read THIS.
Milky Clouding of the Skin – This is OFTEN a low pH. Test that theory and buffer the pH to clear that problem.
Alternatively it could be a protozoan parasite problem for which SHOTGUN ONE or SHOTGUN TWO could be good.