Hmmm
It could be a ballast. The typical lifespan of a ballast is 50,000 hrs of operation but like all electrical devices you may be seeing an infant mortality on the component (i.e. some electrical parts just burn out very early). I believe you have had this tank up and running for 4 months. Not sure if you purchased the tank new or used. But, if new and you run your lights for about 12 hours a day, you most likely have only 1,500 hrs tops on the ballast. It may be the bulbs running out as well, but typically you see it affect one bulb and not the whole set in the front fixture.
Not sure how you have the power hooked up for your light fixtures (direct to the wall or through a controller). Some controllers limit the amperage draw (Apex has 15 amps on its powerbus). It may be possible you are close to the 15amp limit and are therefore being somewhat restricted in your power draw.
Unfortunately, I don't have any experience with the RSM 650 light system since I disassembled it fore LEDs.