New Member: High CYA

Hi to all!
Been reading the site info for several weeks. Have 7000 gallon AG pool in FL. Started using Taylor 2006 kit three weeks ago, when got very high readings for CYA (over 100).
Prior to learning about trichlor here, used the tabs exclusively for about 1.5 years. Don't know what the prior owner used.
Have replaced almost 12" of water over the past two weeks, and it has rained quite a bit... but have experienced an increase in CYA to 160... even though the tabs were discontinued a month ago.
Have been using equal parts of pool and tap water for testing... and repeating the tests... and getting identical numbers.
Been keeping FC to the recommended level, per the CYA / Chlorine chart. Water is totally clear.
Your suggestions would be appreciated.
Thank you.