Convenient to dose the drug
Since we know precisely the volume of the aquarium, we can control the concentration of the drug in the study. We can also control the time of drug exposure. When working with worms and flies, there is no such level of control. In addition, the testing of drugs on worms requires concentrations exceeding the physiological doses for humans by 100 times, which is extremely expensive. Flies mostly eat like larva, and it is impossible to control the amount of consumed drug.