Yes, an RDP(reverse daylight photosynthesis) is lit opposite of DT lighting, that is at night. This helps to maintain a constant pH level in your system, otherwise pH drops at night. There are a few drawbacks to running a fuge light 24/7. First it adds more heat to your system, I already have 720 watts on my 55g, so temp is a concern for me(unless you have a chiller). In nature of course macro's are not exposed to light 24/7, so you are not replicating the true environment and giving macro's a chance to rest. But, the biggest reason is pod production. Pods do much better in fuges that are not lit 24/7.