This is something people tend not to consider when they get a used tank for cheap. I've kept tropical fish for years, and in my experience, people sell tanks for two main reasons. They're upgrading and don't want to reset the tank up, or they had a mass die off of fish and decided not to start over. Obviously there are other reasons, but unfortunately I see the second situation a lot. Regardless, this is excellent advice regardless of what you're planning to do with the tank.