I run iodide (not iodine ) at .03-.06......27 years.
Sounds like a water chemistry and stability problem.
What are your parameters, what is the flux,,how long have they been stable.
While BTA can withstand a range of levels, they are not good at change.
It is also very common to shrink for 1-4 days as it expels waste.
Carbon is fine....however some believe it causes HLLE in tangs.....