Sloshing is the movement of fluid in a partly filled vessel subjected to external motions. Generally, sloshing is an undesirable motion for the purpose of marine operations as it causes intense forces and instability. However, the internal motion of liquid might be useful for devices that require mixing. For offshore-based floating photobioreactors, sloshing induced by external ocean wave forces are desirable, as the movement of liquid is able to bring about mixing and mass transfer for the nutrients and gas in the microalgae culture. Therefore, the objective of this study is to investigate the correlation between the regular wave-induced sloshing and mixing behaviour of the irregular octagonal tank. The irregular octagonal tank is modelled as a non-ideal reactor with stagnant volume, and the Residence Time Distribution (RTD) of the irregular octagonal tank is measured. The RTD is dependent on the external excitation parameters, such as amplitude and frequency, besides being dependent on the filling ratio of the tank. Dissolved solids are injected into the system and the concentration of dissolved solids are measured using a Total Dissolve Solid (TDS) sensor. The experimental results are able to show the RTD curve for the irregular octagonal tank undergoing internal sloshing dependent on the filling ratio of the tank, excitation frequency and excitation amplitude of the sinusoidal motion. The mixing efficiency of the sloshing motion peaks at a filling ratio of 30 % with excitation frequency closest to the natural frequency of the irregular octagonal tank and large excitation amplitude.