I had also found some "TINY" cracks around the G1/8" hole on the Plexi Top of Cuplext XT; while the other 2 piece of Twinplex N/B & S/B block's plexi does not having any problem.
I use MCT-5 Non-conductive coolant in my loop.
Thus I assume the Tiny Cracks happened on the Cuplex XT were not caused by coolant, but with the tightening strengt myself. Or else the Twinplex shall have the same situations? ???
Fortunetlly I also bought a COPPER Top Piece while ordered my Cuplex XT Block. Altough I did not notice any leak with my loop, but I changed the Plexi Top with the Copper piece for confidence.......