Brick red is from dissolved iron oxide=rust. Fuel went acidic and started to eat the tank. Probably had ethanol blend gas in it, although non ethanol fuel will do the same thing if left in a tank long enough. If the car ran with that stuff before you cleaned it be sure to keep an eye on the fuel pump for leaks and change the fuel filter if you have not already done so. Degraded fuel is also real hard on rubber fuel lines. Car looks great!
Dave