Koi Angel fish body issue

[Milliardo Peacecraft]

I'm trying to figure out what happened to my koi Angel fish.  Today I noticed that he has a mark on his side and I'm not sure if it's a bacterial, parasite, or physical damage issue.  He's basically the biggest thing in my 75 gallon tank.  The next closest in size to him is a 4-5" bristlenose pleco.  The other fish are black and white skirted tetras, a Denson barb, 3 mollies, 4 siamese algee eaters, an oto.  I'm trying to figure out what I should put in the tank, meds wise, to help, but I'm not sure what is up.

Water parameters:

Nitrates: <10 ppm

Nitrites: 0 ppm

Hardness:  150-300 ppm

Buffer: 180ppm

PH: 7.6 to 8.0

Chlorine: 0 ppm

 

[Colu]

Looks like he's swim into something  possible a piece of wood and stab it's self I would monitor it might heal on it's own with stable water parameters and add a small amount of aquarium salt 1 table spoon for 10 gallons that will aid Gill function and add essential electrolytes if you notice any Reddening or fuzzy appearance to the wound then I would quarantine and up the aquarium salt 1 table spoon for 2 gallons and do a course of kanaplex @Milliardo Peacecraft

[Milliardo Peacecraft]

Well, I noticed 2 days ago that the angel fish wasn't eating at night and was staying back in the corner by the filter in the area that he had claimed as his for the last 6 months.  The next day my 6 year old daughter found him dead on the bottom.  Not the best day for her.  That was her favorite fish in there since he had the most personality in the tank and would follow her around where ever she was looking into the tank along with following her finger all over the tank as she moved it around.