I don't think it's your lights - at least, not from a spectrum-shift (aging) point of view. They're just not that old yet. I suspect it's more a consequence of the immaturity of your biological filtration. That is, you're going through "new tank syndrome" where your biological filter is still maturing and the cyano is just closing the nutrient gap.
In the near term, it looks like you're going to cut the lights off for a few days. Be sure to do a good water change at the end, before you begin easing the lights back on (a few hours a day - not all at once). You may have to repeat this treatment approach two or three times over the next month or two.
Also, you may want to consider a media reactor for your carbon (and possibly a GFO (granular ferric oxide) phosphate remover). The passive bag-o-carbon in the sump is only moderately effective compared to the flow-thru method in a reactor. This will help to remove DOCs in the water column.
Keep up the use of the RO/DI (Reverse Osmosis / Deionized) water for your water changes. Personally, I change out about 7-10% of my water each week, which would be about 5-7.5 gallons per week for you. All of my top-off is RO/DI, too, which probably adds about 5-7 gallons per week on your system, I suspect. You could be consuming, then, as much as 10-15 gallons of RO/DI water each week. Ultimately, you may find it more economical (as well as convenient) to get your own RO/DI setup. You can get a good one for under $200. Sources are many but, include The Filter Guys or Air Water Ice.
If you keep manually removing the cyano film, then make sure you take what you can out of the tank through water changes or siphoning. There's no sense in leaving the nutrients that are captured in the slime floating about. Also, don't disturb your sandbed too deeply (more than, say, 1/2") if you do vacuum it. The deeper, anoxic part of your sandbed should remain undisturbed so it can develop to do its job.
Good luck with your situation.