What are your nitrates? In tank and out of tap?
The pits/white pockmarks on the jaws and sides of the fish look like "hole in the head" to me. Head & Lateral line erosion. Usually caused by poor water quality. Sponge filters are okay if you clean them all the time and if they are large or numerous enough for the tank size. I personally wouldn't run an overstocked large tank on sponges alone, though I know some people swear by them. I would bet that with the two big O's in the 75 and only sponge filters & weekly water changes, plus twice a day feeding, that your water is not getting clean enough, and the fish's health is deteriorating as a result. I would aim to keep nitrates very low to stop the holes from spreading and growing deeper. It can completely disfigure a fish if allowed to get too bad.
It also looks like there are fungal or bacterial infections on the fish, hard to tell which from the picture. If you can get ahold of some melafix or pimafix, I would dose both at slightly reduced strength.