Unknown pest identification

[sculpin2]

Not sure what these are. They move as fast or faster than an amphipod or isopod but are shaped like a flatworm. Predatory isopod, maybe? Locomotion suggests legs/appendages but they are too small for my camera to see. They were all over some otherwise healthy appearing frags from a local pet store. Did not succumb to Coral Rx Pro, although it did irritate them. Size is 2-3 mm. Sorry for the giant pics!

Thanks!

 

[sculpin2]

Here's a video of one moving for reference.

https://imgur.com/ZjXiZdb

 

[sculpin2]

And another one:

https://imgur.com/qXnqPHU

 

[sculpin2]

And the answer is:

Serolid isopod!