I think fish would definitely be among the most abused animal based on how common a pet they are, and how little research people do...
Just yesterday when at first street i saw a couple buying a clown fish for a bowl, yes a bowl!!!
Not to mention all the other large fish like paroon sharks etc that people pick up and obviously have no idea how to keep, even baby arapimas are being sold again which made me quite sad to see since i thought stores stopped bringing them in...
I mean when you consider how many fish fatalaties there are > dogs and cats at the expense of lack of research, i think the numbers for fish would be much, much higher