I would use it after filtration on your Fluval 304. If your water becomes clear and you take it out I would say that the algae bloom might come back, i'm not pretty sure. is the tank near the window? I would suggest to keep it their and use it, besided you bought it already, not only does it kill free floating bacteria it also prevents some outbreaks on other disease. UV sterilizer kills all free floating bacteria and other organisms that maybe harmful to your fish and tank.
I have 11 datnoids, 3 of them are 3 inches the rest are around 4-5 and growing fast. it is bare bottom and running on a sponge filter and xp3. don't have any problem. i will transfer them to the 180 after it gets fully cycled. (but i'm thinking of selling them soon)