Besides, Urchins don't do well in empty tanks....j/k. Water and Vinegar has always worked great for me. Next time, I'll try the cold water technique. I know cold water out of my outdoor water faucet has never worked for me.
When I cleaned out the 12 gallon, I soaked the tank in the tub with hot water and vinegar for about an hour and then went at it with the inside piece of a mag float...Everything came off easy