Yor loss immediatly after intoducing the PVC would seem to point at it as the cause. BUT many systems including mine flow threw PVC with no issues........
I have on mine, about 50 foot total distance of
PVC on a loop system that runs to my basement and
back.
With the large and immediate overnight loss of fish, I would look at possibly other causes or issues being involved . The issues of migration and leaching Joe listed in the info on PVC it seems would only result in long term questions, not a quick loss. Neat read Joe..........
Something like a spike in Ammonia from the loss of the first fish effecting
the rest.
Was the Mag on your system turned off for a while and them restarted without first cleaning it out??? That would be my first thought as the culprit.......
Letting one of them off for a while and the starting again without cleaning can be like pumping sewer water into your tank if it was full of crap.
I learned this lesson the hard way, a couple of years back. I had a big Powerblue in a basement tank with a canister and a GFI.. The fish was going to a shop that week for sale and a nice profit on him. One morning I noticed the GFI was off, before thinking I reset the GFI and the smell of sewer water hit me........ Daaaaaa Lost the Power the next day.
Sorry for your loss.