The normal water quality report doesn't list everything, it just lists things required to test for by the EPA.
Uranium and lead are probably present in extremely small amounts (too little to fail the EPA standard), so it definitely wouldn't cause the clownfish problems since the ocean generally has these contaminants already present.
Clownfish are, however, sensitive to copper I know for sure. But so are inverts, so if you had high levels of copper they shouldn't be alive.
It really is not Petco in general that is the problem. It is the staff they hire.
My local petco had a staff member who was managing fish that had their own nano and sps tanks. So they knew their stuff.
They always complained about how management wouldn't let them do what was proper to keep tanks good.(district managers) So it was a uphill battle just to do things basic and proper in maintenance for them. She even went as far as refusing to sell fish that came in for 24 hours till she was told she can't do it. So she put a warning up instead. She has now left that store and works for a real SW fish store.
Last time I checked they had a FW manager in charge of the salt. The coral tank 80gallonish was covered in redslime and most everything near dead.
I have since had a talk with the assistant manager there and we got them up to speed on maintenance...its still bad but not as bad.