I meant some people no matter how much you prove them wrong and refuse good care. Talking is best but showing a paper care sheet will make your point.
I don't think it's ever a good idea to try an prove people wrong and myself right...
If I talk to someone because I care about them and not just because I want to prove them wrong... it always works out.
There is no point of proving someone wrong if I will only agitate them, I'm willig to take a step back and accomodate their opinions and talk to them from a level they can accept.
So usually instead of:
"Your tank is too small, you should get atleast a 2.5 gallon, or your betta will die prematurely"
I would say:
"Hi! Your fish is so beautiful! it's a shame that the tank's so small! Did the LFS worker tell you it was ok? nah... don't believe them, Bettas don't actually live in tiny puddles *insert fact about real betta habitat here*.
I would recommend you to get a 2.5 gallon tank. It's also a bonus that they're easier to heat because bettas are tropical fish, and don't do too well without a heater! Did you know they've been known to live up to eight years?" etcetc.
Ofcourse the above conversation will be modified according to the listener's response :) 99% of the time, it gets the message theough to people XD