I just cleaned and set up a 20 long I got for free, it was in just as bad condition as the one you picked up. I found the easiest way to get it clean was fill it with a tiny amount of water and use a razor blade. The water was to dip the blade in as it rinsed it and seemed to help cut through the harder spots. It took me some time but I didn't have to use any chemicals or buy anything and it came out crystal clear.