your sump size would need to be rated on how much water would be siphoned out during a power outage to not overflow + how much would be normally running through it while in operation.
in my opinion, you are overstocked on your tank. ID's are messy. i have an 18+" ID in a 300g with a 60gal sump, 10x turnover rate, and it is a right mess. if i didn't have so much media and BB in my sump, i could definitely see having poor water quality.
they also do better in tank with more space. their ideal tanks allow for them to really swim in, not just bounce back and forth. i feel bad for mine in a 6' long, 4' wide tank.
a sump is a godsend, the amount of media you can place in it for BB colonization is far superior to that of any canister or hob filter. add in the mechanical filtration/prefilters and you're laughing. place your heaters in it, and you have more room in your tank.
as far as the markings on the side, it could be bacterial, which irritated the ID, who then was rubbing itself on the gravel, and scraping itself doing so.
gravel can cut scaleless fish up much easier than sand can. gotta remember that. remember, that just like humans, not every fish has the same constitution, some get sick easier than others. can't simply say "well, he's the only one..." give it enough time and other fish would get it too.
keep up the water changes, add more filtration for the time being.
i'd look into removing the gravel with something more scaleless friendly.
eventually you will have to upgrade the tank if you plan on keeping the IDs longterm.
