I just got 4 Serpae tetra two Otocinclus and a dwarf red gourami one of the tetra is nipping the gouramis tail what can do?

Maybe the fish have been starved of protein. That is a common cause of fin nipping. Try feeding the fish some live food like Brine shrimps (Artemia). If that fails then the offending fish should be placed in a small container and floated in the tank for a few days and then released to see if that has changed its behaviour. Don't forget to replace its water several times daily though.