#2 diesel fuel needs no additive with possibly one exception. Diesel fuel comes in two versions- Summer and Winter blend. Winter blend is a lighter blend to prevent gelling. If you fill your tank with Summer blend then are going to park it until it gets very cold you "could" have gelling issues. In that case adding a anti-gel agent when filling the tank would possibly prevent a problem later on. Adding anti-gel agent to Summer blend in the tank that has gotten cold is probably too late. It needs to circulate and mix with the fuel.
Jim