Ok, I can finally get a pet BUT?

My Gramma FINALLY said I can get a pet besides that dumb, old, evil, annoying, untrained 15 year old dog. My choices are gerbils, hamsters, or an aquarium. But here's the problem. Ever since I had these hermit in third grade, she has been very paranoid about animals. The reason is because I didn't clean their cage well enough. I took care of them really well but i didn't clean the tank well enough and they had mites. So long story short the animal has to be kept outside in the garage. I really want a gerbil but if she makes me keep it outside that's abuse to it and it will die of suffocation if i can't bring it inside at night. So i'm thinking of getting tropical fish this may when the weather gets warm and get older ones so they will die by fall. I'll get them a heater so if it's too cold, they will be warm and if it's too hot, i'll put a fan on them if they don't make conditioners. Any ideas for the gerbil or hamster? and some tips on the fish? Yes, i know my gramma is an idiot. P.S I live in Illinois so the weather can be pretty warm in the summer but it can be 60 degrees on a bad day,too.