[DrGonzo]

Try Muriatic acid, no scrubbing involved. I cleaned a saltwater sump with it when I converted it to my 90 freshwater. Looked brand new when I was done. 1 gal bottle is like $8.

 

[DrGonzo]

If you plan to go this route, please take the necessary precautions as concentrated hydrochloric acid is quite corrosive. The fumes are also quite dangerous.

Very true. Gloves, goggles and plenty of fresh air are a must but you get the desired results. Rubber maid tub in the garage works fine.