We are having problems with the tile grout in our shower. The shower is all black tiles with white grout and the grout is either completely gone or partially gone in isolated areas of the shower. About 1/4th of the grout on the floor of the shower is gone. The tiles are still intact but we are wondering if we should have a tile person come in and completely redo the shower floor or could we just add new grout? If we can do this ourselves, is there a good grout that we should use that could withstand water and strong tile cleaners?