This happened to me too...I've never had a hamster before and did background research and stuff, but I didn't really know how big the cage had to be...I was advsied to get a cage- a Peggy Metro by Savic, which, to be honest is a pretty bad cage, but then I found out it wasn't actually big enough for 1 syrian hamster, so had to by another small cages as an extension to it.
Same with the fish-my goldfish has a tiny tank that I thought was an ok size, turns out its really bad, but I can't afford a new bigger one so yeah