Ni-struvite crystals formed fast within t < 100 s, while Co-phosphate octahydrate crystals were firstly observed after t > 600 s. Although we observed slightly contrasting kinetics among the used approaches (e.g. see time axis in ESI: Figure and (Ni) S8/ (Co) S9), the differences can be explained by changes in the hydrodynamic regime between the setups (confinement, pumping rate, etc.). We can conclude that Ni-phosphate demonstrates significantly faster crystallization/transformation kinetics than Co-phosphate. However, both elements precipitate into analogous struvite crystal structures, at least at low M/P ratios, although the pathways are clearly not identical. The Co-system is potentially affected by the presence of long-living metal clusters, indicated by the long-term plateau value in pH at 5.7 and the delayed color change of the solution (Figure , ESI: Figure ).