Some people will never be educated on fish health and welfare
You can explain things in simple terms til your are blue in the face but they will take absolutely no notice of you. Most like this will retort that "the person in the shop said it was OK and they know what they are doing"
As I have said before....a fish will tolerate alot of things but unless their environment is right, they will never thrive.
Sometimes you have to take a deep breath or ten, count to 100 a few times and walk away from these people. They won't listen so no point wasting your breath.
And when it all goes entirely pearshaped...which it will....you can tell them straight, "I did tell you it was all wrong but you refused to listen....now do you actually want to keep fish properly, if so I will guide you...but if you stray off the instructions, I wash my hands of you"