A drilled tank has holes drilled somewhere in the glass that is used to move water from the tank to a sump/fuge/filter and then back again through another hole. An undrilled tank relys on pulling water out of the tank over the top of the tank for filtration and such.
Water top off is when the water in your tank evaporates, and this happens pretty quickly, you need to add water back. The water you add back is fresh RO/DI or Distilled, not salt water. When the water in the tank evaporates it leaves the salt behind and this increases the Sg lever. A top off with fresh water brings it back into the accepatable parameters.
