joisy:
Good question. I recently constructed a tank farm with a complete fire fighting system and we utilized plastic tanks because of the AFFF corrosivity problem. Unclesyd is right, you should check. CS tanks are not recommended for long term storage because of the airspace over the solution. If I were changing concentrates I would have this question too and if it was not readily answered by the vendor, with technical backup, not just his word over lunch, then I would require a performance test with the existing proportioners to be certain. Actually, this should be readily answerable by either the foam vendor or the original proportioner vendor, one of the two.