What an advantage to have an auto top-off device! I'd say utilize it! As Paul was saying, top-offs are not a big deal with FW tanks. I like to let my water sit overnight if possible, just because I think after I've treated it, it still needs a little time to age and release icky stuff. But that's just me. Plus, then with the dechlor the TDS tend to conglomerate and sink to the bottom of the bucket in our incredibly hard water.