There are a few factors you should consider before claiming a pet on your taxes, and that includes their conditions. You can write off pets while claiming taxes only if they're adopted(which I don't think it's the case since you can't really adopt a fish, lol), business animals, if they have any medical issues, if you put them in your will or if you're making a business off of them(such as a YouTube or Instagram account with thousands of followers). If your fish enters any of these categories, well, I think you're good to go! If your fish is a therapy animal, it can also work out!