I'm not that experienced with Ariods specifically so this is general plant info. Since the interveinal chlorosis and scorching is on the older leaves and not the newer ones it would suggest Magnesium or Potassium deficiency. Both deficiencies are common on sandy soils. Since the 10-10-10 contains Potassium that seems unlikely to be your problem. However, excess Potassium can interfere with Magnesium uptake in plants. A soil test would be best. Make sure they test for Magnesium levels.
