Obviously more light never hurts, but since you have to buy the envirolight, it's not necessary to do so. Now, that may depend on the size of the MH you have. I'm assuming it's 400 watts or greater. If it's a 250, the envirolight would be a good idea. (Although there are other options of course.) FYI: If you can only have one HID light for an entire grow, Metal Halides are better overall than High Pressure Sodium. Don't let anyone tell you any differently. Although it's true that HPS are better suited for flowering (as I mentioned earlier) MH bulbs have a fuller spectrum and are therefore preferred for a full grow (if only one choice is available.) Let me explain it another way: There is a fair difference between MH and HPS when it comes to growing in the veg cycle - MH being significantly better for that period - but HPS do not outperform MH in bloom as much as MH outperform HPS in veg. Know what I mean? Ultimately, MH is the choice if you can only have one HID. Again, don't let anyone tell you any different.