Cruelty to animals. I eat meat but do not like seeing anything suffer for purposes of amusement. These are small, but mistreating even insects can be a crime in the US (specifically if you film it). Give them a digital version and leave the actual animals out of such displays.
The creators of this project told me they actually wondered if PETA would take offense. Ideally, though, the tardigrades would have had plenty of algae and room to swim around/reproduce.
Just in case you're not joking I'll point out that water bears do not suffer in any meaningful sense because they have far fewer neurons than even a mosquito.
