Oh no, first I tell you how I am trying to save a little duckling, now have to tell you that the duck just died.
 
I have no idea why. He was in the bathtub for his daily swim and wash. Its been a ritual for more than a week. The tub was only filled enough to allow him to swim at the deep end and fully stand up at the shallow end . I leave a video record ON every time he takes a bath, then I leave him alone because he never liked me being around.
 
The video shows him washing and swimming back and forth then about a half hour into the video he started swaying back and forth as if he were dizzy. Then he just layed over on his side.  I found him upside-down under the water when I went in to bring him out.  I didn't watch the video to the where he actually died. Me and June decided to cut away from that portion.
 
How sad this is. I was really looking forward to watching him grow and letting him loose. Now I'm wondering if I did something wrong.

 

Oh what a shame but you did your best to rescue him.  Maybe he had an issue all along that you couldn't possibly know about. He's at peace now in duck heaven!

Aww, poor little thing. But at least you and your wife tried to save it and gave it as good of a life as possible.

I googled caring for orphaned duckling then mallard duckling. Might? have gotten cold without mum's and siblings body heat, not having developed waterproofing of feathers yet. 

So many results and websites which I'm sure you already researched/video tutorials. Not sure what temp. the tub water should be. One webpage said only a bowl of water but could have been a different type of baby duck.

I read about a heat source in the box, some said 60 to 90 watt bulb, a cloth covered hot water bottle or covered heating pad and I really liked the mirror in the box idea so little one wouldn't feel alone.

It would seem hard for a lone duckling to survive without being under mother's wings and sibling brood. Also read she has the duckings swim for short periods then brings them (they follow) out of the water and mother duck sits on them to her breast warming them up.

Weird.  Poor, little guy...but don't take it too hard, you were just doing something to help and there may have been other things going on with him.

 
Yeah, he/she had two mirrors in its environment, a heating pad underneath at one end, I have recorded sounds of other baby mallards chirping around their mothers, as well as other mallard ducks calling each other in the distance that I played and looped on a regular basis to let him know he wasn't alone, and of course the proper food for its age with the proper protein to allow its wings to grow properly.. 
 
I did a lot of searching on Google and YouTube to see what was right or wrong for a baby mallard and thought I had all the bases covered. He was growing so fast, always cocky, and always ready to fight me any time I came near.  Which meant he was staying wild and not domesticated.  I was really looking forward to watching him fly off in another 5 weeks.


 

Amazing they can fledge in 8 weeks! You did an amazing job, above and beyond!! Other than becoming a mother duck, no stone was left unturned and as others & websites said, must have been unknown or underlining factors. The little guy/girl was so cute, RIP little one. I saw the mirror in the pic below, he/she sure was flourishing and wow you had two mirrors plus the mallard nature sounds. proper diet, environment  heating, etc. etc. ....nothing more you could have done.

So sad.....but you gave it your best.