well, if it's persuasive, you'll probably want to make the topic more specific. Or at least the sentence describing it needs to be more specific. So like, you are trying to persuade people that petstores do not provide adequate care.
Now you'll need to think about the arguments people could have against it.
First, you'll need to provide proof that the care is bad, using examples of wild bettas and disproving any myths about them.
Next, people will wonder "But how can a few days in a cup hurt? They will go to a new home soon right?"
So you might provide examples of a betta living in a petstore for like a whole month or more, AND then you now have the ability to talk about petstores giving false information to customers in order to sell the fish.
They may ask, 'why would they provide false info, big tanks are more expensive, don't they want the money?'
You can now describe how they are labeled as a beginner animal, and that the majority of uneducated owners would not buy a betta if they knew it took all that money. You could poll petstore people asking them how much they would be willing to spend on a betta, and ask them how they feel knowing they would possibly need to spend 100 dollars to properly house one. Now you can say how this would affect the business of the store.
I know one person mentioned on here that their walmart basically has insurance on their fish, and that they at least get some of the money back even if the betta dies. if you could find info on that, there is total proof that they have no reason to care for the fish!
Asking petstore workers about it will help. You can find workers and ask them about if they even think the fish are getting good care.
:D Those will be REALLY strong points!! Top it all off with stories of long living bettas and healthy strong fish and you have an excellent start to the research needed!