Before I paid $200 for a new tank, I'd put some dilute Muriatic acid/water mix in the tank and let the acid do the work. Muriatic acid is available at most hardware stores and has to be used with caution, it will burn anything organic it contacts. Also, it's used for etching and cleaning concrete so don't use it in a garage with concrete floor.
There are very few things, I've found, that can't be cleaned with Muriatic acid. After the algae is removed, rinse the tank thoroughly and if paranoid about acid residue, throw into the tank some baking soda and water.