Yes, the soap will eventually wash off.A good rinse and dry it in the sun should do the trick. In fact quite a few years ago there where some "how to" books that came with aquariums that recommended using a mild dish detergent and rinsing it thoroughly before using the new tank. Allthough using detergent is not recomended, just using it once or twice to wash a tank "shouldn't" have caused long term damage. There may have been other reasons for the fish deaths then the soap. If this is a new set up I would reccomend patience and do a fishless cycle to establish a nitrogen cycle. This way you can add fish at virtually no risk of "new tank syndrome" death.