Okay...the tank will be okay in terms of space...it is a bit tall...usually most people use tanks that are long...in measurement they are called 20 gallon long, thirty gallon long, etc...which makes them longer in length...but the snake should have plenty of room in the tank you have there. I'm not sure about the plants and stuff...my concern would be the fact that they were in a fish tank before...I don't know a lot about it but I don't know if bacteria or diseases could be transferred...your best bet is to get some fake plants from a dollar store...if you are looking to save money and you don't want to buy stuff at a pet store. The cardboard boxes and stuff that you already have are fine...if that is what the snake likes, go with it. You need to be careful with your bedding...you don't want anything the snake can ingest...best to get something made for snakes like aspen or carefresh. Also...I don't know enough about lime deposits to know if that would be any risk to the snake...but you said you are cleaning it up...just be very careful with what you are using to clean the tank...make sure you get any traces of cleaner out of the tank and air it out properly before putting the snake into it. If any other more experienced members can comment on my advice..please do...I'm fairly knew to all of this myself! Do you know about hides and lighting and all of that?