It's up to you really, am not encouraging you to break your lease however I have a hamster in my uni room that isn't meant to be there. What I would say though is compared to something like a hamster a fish tank is a lot harder to transport when you have to go home and personally I would be worried about the tank breaking and soaking your room.
I simply put my hamster in the wardrobe if there is a risk of someone coming round (like you my accommodation emails beforehand) as they aren't allowed to go in your cupboards but I feel like a massive tank would be a lot harder to hide tbh. You also have to be conscious about water noise from the filter that anyone inspecting could hear, my hamster just sleeps in the day so there is no risk of him making any noise.
For me personally the small risk of getting caught (small in my case) is offset by how good my hamster is for my mental health however personally I think fish would be a lot harder to keep hidden...