problems with jack dempsey sichlid!

[cup o noobles]

so when i returned from college a couple days ago my dad showed me that my 10" jack dempsey cichlid had a huge gash/hole on its right side. it was very lethargic and wouldn't eat any food. My dad said that the scales had started falling off and then the hole just grew and grew every day. the jd looks pretty bloated on the underbelly too and now it will eat some food and spit it out immediately. I checked my water and found everything wwas where it should be but nitrate level was 200+ (I guess my dad didn't take the best care of my fish while I was gone) so I did a 30% water change and plan to continue changing the water. Also I started using melafix to help repair the hole but the gash seems too big for some tea extract. is it more important to change the water and dilute the melafix or should I just hold off on water changes til after the treatment is done? and does anyone have any idea what caused the gash on my jd cichlid? could high nitrate levels cause that? thank you!

55 gallon tank
7 african cichlids plus the jd
penguin biowheel filter
I've always used cichlid gold pellets