First off, welcome to the group!
Being in the city, I think your rat friends are going to keep on coming, no matter how much "food" you get for them, BUT I would not give up!
You dont have a lot of space to work with, but you really can make that small space a WOW factor for yourself, and those walking past ...
I would give up having a liner given the rats, but instead, go with a 150 gallon Rubbermaid Stock tank ... a lot bigger than what you had, but you do have the room ...
http://www.hardwarestore.com/stock-tank-639394.aspx dimensions are included on the page ... To this, I would add a small, plastic river/waterfall ...
Being inside the city limits, do you have any restrictions regarding height of plantings???