I question the validity of some of the information on the site posted above.
Here are more reliable links: The Horse: What Is Stringhalt? The Horse: Classic and Australian Stringhalt
Yes, removing from the paddock and inspecting for plants that may be causing problems is a good idea but going to hard feed is not necessary or necessarily in the best interest of the horse as their digestive system isn't designed for high concentrate diets. Providing adequate good quality forage and supplementing with a ration balancer would be the best choice while keeping the horse off of pasture.