Which ligand names have to be parenthesized while naming the Mononuclear Coordination entities
Greek prefixes are used to designate the number of each type of ligand in the complex ion, e.g. di-, tri- and tetra-. If the ligand already contains a Greek prefix (e.g. ethylenediamine) or if it is polydentate ligands (ie. can attach at more than one binding site) the prefixes bis-, tris-, tetrakis-, pentakis-, are used instead.
Bottom line - parenthesize only those ligands which already has a greek prefix in its name so that you can highlight the use of the alternative prefixes such as bis-,tris- etc.
Ps -Diaquabis(ethylene-1,2-diamine)nickel(II) sulphate is the correct naming but you can make it more appropriate by using ethane instead of ethylene