That really depends on what type of tank it is and what the levels of the newly mixed SW are. As a general rule, no you do not need to add any buffers to your newly mixed water. For my reef though, I have to add calcium chloride to get the CA level up and bring the alk down. That is all I would be concerned with, if I were you, there are plenty of traces in the new water, just test it for CA and ALK, buffer if necessary. If you have a FO or FOWLR, it probably isn't necessary to buffer or even test.