SWRO Permeate

I am an operator and have SWRO permeate produced in Equatorial Pacific waters, that isn't getting remineralised/reconditioned. I am aware of RO permeate not fit for consumption generally, but I have also worked with RO's that generally do remineralise/recondition. Severe corrosion is occurring in and on the steel storage tanks that are lined internally, but it is difficult to determine whether this is just the seawater salty environment as this island is very small or the RO permeate. 1 out of the 3 SWRO's I operate has a Calcite filter but increases pH to approx 9.10, so acid gets dosed to control pH. Can anyone give me an easy description of the Langelier Saturation Index and your professional opinion on whether the calcite filter is required, obviously without a feed water analysis? I believe it is. Thanks