Whats wrong with my little vlamingi tang?

That's my little guy, he also has fin rot and tail rot as you can see. At first I thought my little damselfish was picking on him, but I don't think a fish could nip at his side like that, and it's only on that one side. It looks like he is missing skin, but only on that side. it's been there for at least two months now, i didn't think much of it because he was acting completely normal. And vlamingi tangs change their colors and have spots when the light changes or anything, so it's right along that line where he looks spotty when he changes his look.

I started a melafix treatment 5 days ago, and have two more to go, i see it sort of helping the fin rot, but not his side. I also changed the diet for my fish just for the tang, with more brine shrimp and spirulina, the flakes, also with spirulina and garlic.

I do water changes at least every two weeks, if not once a week.

My readings are salinity at 1.020 which is low, i know, but my cardinalfish like it like that, i might try to slowly raise it up to 1.022 this weekend, when i go to buy some RO water.

Could tap water be the problem, also?

My nitrites are 0.25 ppm today
pH - 8.0-8.2 ,
Ammonia - 0 ppm
Nitrate - 0 ppm

any advice on what that booboo can be, and any help on how i can help him will greatly appreciated.

Do you have him in a 55 gallon tank? That's part of your problem. That tank is way too small for a vlamingi. They are one of the tangs that need the most swimming space. You also have 8 fish in a 55 gallon tank, which is overstocked. It's especially overstocked if you are keeping large fish like a vlamingi.

The marks on his side are called HLLE -- head and lateral line erosion. It's caused by malnutrition. Tangs are herbivorous. They need a diet filled with vegetables, mainly seaweed sheets. If they live off of brine and flakes (like you have been feeding it), they will become malnourished, get sick and die. It looks like a pretty severe case that your fish has. HLLE doesn't pop up overnight -- it takes a while to get that sick. The fin and tail rot is another issue -- overall, your fish is in pretty bad shape, and the combination of being fed the wrong things and being kept in a tank that's too small isn't doing it any good.

You need to buy some seaweed sheets and start feeding them to the tang daily. Brine can be a snack, but for any fish, even carnivorous fish, brine has very little nutrition.

You need to bump up your salinity to 1.025 or 1.026. Natural seawater has a salinity of 1.026. I don't know why you say your cardinals like it lower. They really don't. Saltwater tanks do better when the salinity actually matches the ocean's salinity.

Lastly, you need to find the vlamingi a new home, or get a bigger tank for it. You can do everything right to try and get it back to health, but it will never be healthy in a 55 gallon tank. That's like keeping you locked in a closet your whole life.

Sorry to sound so harsh, but this fish has several problems and they are human-caused.

not harsh at all, i have realized that it is my fault. i do feed him the seaweed sheets, every other day and sometimes everyday

He is in need of some TLC,thats for sure.Hlle is tricky,there is no one think that makes it show up...heres a great article
Marine Head & Lateral Line Erosion: A Description of the Syndrome and a Review of its Speculated Causes by Steven Pro - Reefkeeping.com