The magnetic field effects on the picosecond photo-dynamics of two different nickel(II) centered tetrapyrroles dissolved in coordinating solvents are compared. These compounds exhibit fast intersystem crossing processes with multi-exponential kinetics which are altered by external magnetic fields of some teslas. The influence of molecular symmetry on the magnitude of the field effect is investigated and treated semi-quantitatively using a stochastic Liouville approach. Modelling of the effect allows to estimate the zero-field splitting of very short-lived triplet states.