I certainly hope that you said something -publicly- to the person who helped you at the store.
Generally, the more people who hear you say something like, "Sir, I think that tank has ---, look at how the fish ----. Has the issue been addressed, and why aren't they quarantined?" the more likely they'll be to deal with it.
You can't be too "oh, poor fishies" or "what the hell is wrong with you people to let this happen?!" about it. Just a calm (and clear-spoken) "Reassure a customer who doesn't want to spend money on disease, please"
As to your mother, have you tried being sneaky? Change her water for her, a cup at a time, and pretend it's just force of habit. That's how I dealt with my mother's plants for a good while. Once she saw the difference (and, admittedly, had the peace of mind) she got right into it. Now many of her plants are thriving better than mine...
I'm sure that she cares more than you know. But most of the time, the older we get, the less we wanna hear people tell us we're wrong.
Just be respectful and laid-back about it. She's an adult, she's going to make her own decisions and you've got no more say in those than you do in who she kisses goodnight. But you can always throw out what you've learned. People react much more intellectually to ideas that you present for review than those that you hand them as concrete fact. Every person has the right to process information and decide what they believe in.
My mother and I bounce ideas and information off each other constantly.
"I read an article today that stated ----- "
"Oh? Well I guess that makes sense, but ----"
"That's what I would think, but ---"
"That sounds a little half-baked. Did it say anything about ----?"
"No, but it cited ---- study, and gave ----- reasoning"
And so on....