I live in Houston where it can be both very wet and very hot. In our neighborhood the water tends to gather on the side of the road, from storm water and sprinkler systems, but most evaporates with the hot sun over the days ahead. There are some puddles that stay for a longer time (2-3 weeks) as the drainage in those areas is not too good and the puddles are a lot deeper. A bunch of frogs had a croaking night a few weeks ago and 3 puddles were full of tadpoles the next day or two. To cut a long story short, the rain has been pitiful and these puddles have almost dried up. Yesterday I was concerned about one of the puddles that had 1000 or so tadpoles running around, so I rescued about half of them into a cool bag and transported them to another puddle closer to home. However on visiting the first puddle today all the remaining tadpoles are dead. Any idea why? Do they miss their friends who were moved out?

Separation anxiety and loneliness is especially hard on Tadpoles and more much prevalent than people realize. It's possible this is what happened in your case. It's hard to be sure though, as there are other possibilities.

Let's not forget the mass-tadpole-serial-killer of the 1980s,has never been caught, and suspected to now reside somewhere in Texas.

You may have unintentionally infected them with COVID-19 during the rescue.

Or, it could be, sun hot the puddle a little to hot.

We may never know ; )

If you're lying about being in the Badger state, and are really in Texas nowadays (ex-Sandy Eggo), I would suspect your real name is Journeyman Has A Pond (JHAP).

At the very least, you've got a similar combination of genes. Not that there's anything wrong with that.