I'm planning to eat my brother's goldfish.
Yikes, why would you want to eat your brother's goldfish? Does he know about this?
Yes, but I still think it's rude of me to eat his goldfish
Eating a pet goldfish isn't appropriate and is an odd thing to want to do. I'm not sure if you're planning on doing it for entertainment's sake, but I'd suggest finding something a bit more productive.
I'm going to ignore your advice and eat the goldfish. I wish it was bigger.
I think you should think a bit longer before you go through with this. Evaluate what is leading you to want to do this and perhaps eat a fish a that you purchase at the grocery store.