Use some myratic acid on them, pour it on the dried coraline algae or calcium deposites and you don`t have to scrub it either,do it outside of your house it does not leave any residue or smell when rinsed out and they can look almost new,If you have bulkheads on the tanks remove them before the acid wash,they will dissolve if you don`t,for cleaning those,Use a watered down solution,like three gallons of water and two or three cups of acid,when you see a medium size stream of bubbles coming off of the bulkheads you have it right......Sorry for high jacking your thread ,People who might be interested in buying your tanks would more than likely want to know how to clean them out,To make them look new,Without a lot of work,or expence