I've seen that too many times before when I had my old 100 gal with 1000's of Z's and P's. I've seen with my own eyes colonies get ravaged by Ampipods about 3/8th or an inch or so. really large ones.
Made my old tank into a wrasse tank full of Mystery, flasher, Lineatus, a bunch of leopards, 6line etc. They destroyed that Ampipod issue quick and fast. also did a treatment of interceptor to wipe out anything that was left. ( but had to remove all other inverts I wanted to keep alive out of the tank while I treated the tank ). Now I always have wrasses in the tank with any Zs and Ps