Impossible to say for certain, but I suspect it was the heater that did the endlers in rather than any toxin on the introduced plants--if only because the shrimp are still alive.
While endlers can survive a staggeringly large temperature range, they don't tend to do well with rapid shifts. I wiped out an entire 10g tank of endler hybrids I was developing a few years ago--very similar scenario.
I assumed at first that there must have been something that contaminated the heater--perhaps the store had been sprayed with insecticide or such--but only the fish were affected; the snails and shrimp were fine. Not a single die off, but every endler was a floater. Nor did the heater show any sign of electrical malfunction later when I tested it. What it did show was the ability to raise the temperature of a 5g tank incredibly fast.
I'd added the heater the night before to let it sit the required period in the water before turning it on and the tank had been running between 68-72. First thing in the morning, when the tank was regrettably at its lowest temp, I set it to 78, plugged it in and left without doing more than ensure it was working and not electrically shocking the tank. Best I can figure, the water probably rose 10+ degrees in under 2 hours.
Now when I add a heater--to any tank--I make sure it's first turned on while I have time to babysit it. I'll check the tanks current temperature, set the heater just a degree or two above that and then once every few hours I'll bump the heater up a degree or two.