dried cement like coraline removal hints

[lockrookie]

well i picked up a razor scraper and got all the crusty stuff off but now there is a residue that is still there im sure i wont seeitwhenwater is in the tank but the razor blades wont seem to get rid of it ..will try soaking in vinager already tried scrunge.. i really shuldnt be so picky i think i may just turn this tankinto a salt mixing station. more so if i cant get it to come clean.