.02 is not really a significant drop but am wondering is this the first drop or are you seeing this often ? Your running an ATO so you shouldn't be seeing much change. Usually it is an increase not a decrease if top is not maintained consistently . Most people experience an increase tho due to evaporation or from doaseing two part. Hm? Might want to keep track to see if this a usual reaction. Got me scratching my head for sure. Are you getting large amounts of salt build up in your stand and surrounding area? Do you have a window A/C unit near the tank ? Things to consider???