Thanks for the comments!
I haven't had to trim the riccia yet in the 2 1/2 weeks the tanks been setup, but most of the riccia is on two flat, but large pieces of slate running through the middle of that "valley" if you will, so I'll probably very carefully lift them up and out, trim and put back in.
I believe the riccia is the regular size Riccia fluitans. Here's another pic with the background lit up a bit.
