My two cents is that if it in fact is a quarantine tank, lights are not necessary and gravel is for sure not needed. Bare bottom is better because it is easier to clean than gravel. As far as the black on the fins and mouth...the only experience that I have with fish with those type of issues is from overbreeding and bad bloodlines. Like I said though this is just my opinion. Good luck with treatment I hope they pull through for you.