Thanks, y'all. Every little bit helps.
I just had a thought: I changed Taco's water yesterday, never varying from my usual routine. This includes using aquarium salt.
I just read up a bit on Fish Dropsy. It would seem that it's the failure of the kidneys, which leads to a buildup of fluid in the body. Given that salt, in humans, leads to a buildup of water retention, could the same be true for fish? I mean it sounds silly, since fish live in water, but it was a thought I had.
Taco hasn't changed at all. Given Taco's symptoms and behavior, it definitely looks like dropsy.
More updates to follow later this evening.