Modular ballasts of an aquarium comprise a lampshade and a plurality of ballast modules, wherein a reflection plate and a plurality of lamps are mounted inside the lampshade. At least a socket locates on the backside or the top of the lampshade for holding the ballast modules that comprise housings and ballasts. The ballasts are fixed to the housings and the ballast modules are capable of coupling with the lampshade. Because the ballasts are coupled with the housings, the manufacturers are capable of assembling the ballasts efficiently and the consumers are capable of changing the ballasts simply by pulling out the ballast modules by themselves. Consequently, the consumers can install the ballasts by themselves so as to save the required time for the change adequately.