If it were a fairly sudden change, a few days, then I would say it is probably a bruise from slapping the tail against lily pots, rocks, skimmer, or...
You are probably on the right track. I have seen a lot of people treating fish with iodine or hydrogen peroxide as cleaning agents and then covering...
Im definitely going with three inches, it'll help me sleep better at night. Im going to just build the filter for now and worry about how to get the...