Probably not oxidation, however the product's ingredients could be slowly reacting. Is your tank Stainless Steel, or non-metallic, if not perhaps reacting with sidewalls of tank.
Have you asked your UV coatings vendor...have them check their retained sample from the batch to see if it's happening there as well.
Exposure to light (enough to cause slow curing to begin) could contribute. Does the coating when applied to the print look different then previously run jobs. The slight discoloration in the tank may disappear when actually run.
