Demin Water quality

We have 2 GE-6B dual fuel CTs that rarely run on fuel oil. We have storage of 100k gallons of demin water for water injection in the event that we do start. We are black start units, so we need to keep that much available.

Question: What experiences have others had with the quality of the water being stored for lengths of time? We are going to start testing but are there additives for the tank. Any input would be appreciated. Thanks.