As long as your light is putting out UV-C light it will work. The question is how effective will it be. There are two ways that you can go about this. One, mount the light in the tank and hope that the water coming into the tank will stir it enough to churn up any baddies living at the bottom. Two, mount the light in a smaller chamber that the grey water goes through before the tank. The water flows through the chamber and gets sterilized before getting to the tank. The smaller sterilization chamber would ideally be designed to make the grey water pass under the light in a thin curtain to maximize light exposure. The disadvantage of approach two is if the tank has micro baddies then they would be free to propagate there. If you use some bleach in the tank when first setting it up it should be good. If you don't like bleach, try hydrogen peroxide that they sell for gardening. They sell it to oxygenate soil so it won't harm plants.