OH, I think i know what you did.
You drip acclimated a new specimen, and the drip acclimation used up 2 gallons of tank water and the auto topoff compensated for the loss of water by adding fresh water.
Right?
In that case, do what meozerr said, just make the water slightly saltier than you usually would next few times you do a water change.
It'll porbably take like 2-3 WC's.