Gdog,
The 3" chlorine pucks are most likely Trichlor and for every 10 ppm Free Chlorine (FC) that they add, they also add 6 ppm to Cyanuric Acid (CYA). If your current CYA level is at 90 ppm, then if you do a half drain and refill you should be at around 45 ppm CYA which is fine -- you shouldn't need to add more CYA. However, doing a drain/refill to lower your CYA is not the only option you have.
Certainly, you should switch to an unstabilized source of chlorine (i.e. a chlorine source without CYA in it, so NOT Trichlor or Dichlor) such as chlorinating liquid or unscented bleach. That will prevent your CYA from continuing to rise. If you do not lower your CYA level, then you can continue to use your pool but have to either 1) maintain a higher FC level of at least 6.6 ppm at all times (so higher at the start of the day since chlorine will deplete with sunlight) or 2) use a supplemental algaecide to prevent algae such as a weekly dose of PolyQuat 60 (a more expensive alternative would be a phosphate remover).
I just wanted to give you options so you don't feel in a rush to do a partial drain/refill all at once. If you are already experiencing algae growth, then that's another matter and doing a partial drain/refill will make it much easier to fight an existing outbreak.
If you do a partial drain/refill, then some other water parameters will also be lowered depending on their value in fill water. Most likely, your Calcium Hardness (CH) level will get lowered, but with an AGP that I assume is vinyl, this lower CH level shouldn't matter.
Richard
