Your Danio is a gorgeous litte fashy, and assuming she's a female (looks like it, but I didn't flip up its skirt muhself) a few things could be going on. First, it's been three weeks, that may give us the answer, probably should've laid the eggs in this period if she were going to. It may be dropsy (edema, usually secondary to heart or other organ failure). Some folks swear that a pea (mashed) works as diuretic and can release the extra fluid if you can force-feed them in a quarantine tank. It worked on one of my neons who got quite the belly. Any changes to date? If she's stable and staying in good health, not showing signs of stress (surface-dwelling, gasping, side-lying) or you see her lounging frequently with a remote control in their right fin...then I'd just chalk it up to being a fatty.
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