I don't own an Index pool so I can only go by what I've read. Someone else can / should confirm. That said, I've read the OEM filters are barely adequate for regular use. I would suspect you need to run it for a lot longer than just 4 hours. You said, "Ch was off the chart". Do you mean FC? Calcium hardness (Ch) isn't an issue on a vinyl pool unless it is "off the charts". That would be about 10x more than 20-25. Please clarify your post.