FISH DEAD IN TWO HOURS.
Did you watch the fish? How did it act?
What is the pH, hardness, ammonia ect. ? Any friends have plant tank test kits?
Oxygen, co2, iron, phosphates, nitrates etc.
The only way to solve the water mystery is a water co. test. They can check for
everything.
Since the liner is old it should be OK but a piece in the bowl test will tell.