PO4 could have killed the SPS at .5, but would not have effected anything else (except for stunting growth as stated). Could have gotten RTN randomly as well. The PVC glue would not have affected anything either, assuming you were using the proper, potable cement. Something else is going on. Why are you taking water samples to the LFS? Do you not have your own decent quality test kit? If you're having someone else test your water, you have no idea on what your levels actually are. It's not above a lot of LFS's to fudge your test results, and tell you everything looks great when in reality it doesn't, if it means you might walk out the door with a new creature.