I don’t care how stupid you think it sounds. We need to start educating by being interested. Stop patronizing people! I don’t feel like I should have to say this, but evidently I do. You’re not superior. You are just more informed on a topic or have an experience they don’t.
Rather than telling someone their fish is going to kill a tankmate. Because the answer you’re going to get is “They’re fine. I’ve never seen them fight.” Tell them about your pets. I don’t even care if you make it up. “I kept my old goldfish and cory together too… I actually wouldn’t recommend it anymore. They were perfect for months and overnight my goldfish accidentally bit my cory. He got stuck in her throat and they both died. I’d hate to see that happen to yours and have you go through that. I’d recommend rehoming the cories, goldfish are just as fun without them.” I mean you can sound more formal. But you’re not attacking them. You’re not even telling them what to do. You’re informing them that this is a thing that can happen. Ask questions! Don’t assume. Provide resources and stories. Don’t lecture. Most people just won’t listen, you just need to let them go and hope for the best.