My DG died a couple of weeks ago.
The only symptoms i saw were his lethargy and that over a few days he seemed to lose the ability to swim properly. He started hiding in the bottom back corner of the tank, swimming up for an occasional gulp of air, then sinking back down. No discolouration, no change in size or shape...
Then after about 3 days, I got up in the morning and he was dead.
It was pretty sad, and actually annoying that I couldn't find anything online to diagnose properly.
I hope yours fares better.