No tank can evaporate enough water that adding cold FW
to replace it is going to make a difference in temperature unless you went a week without adding. Mine evaporates about 1 gal
a day and even if the water was at 33 degrees it would barely make a dent in the 54 gal
of 78 degree water.
You should replace FW
that evaporates (salt doesn't evaporate) into the tank at least every other day to maintain sg
(which rises as FW
evaporates) and preferably everyday.