I have a 6 gal tank with the finnex ray2. According to their par charts I whould be around 30 or so(light is ~24" from sub). Dosing ei with pressurized co2. So, light and ferts should not be the culprit. This leaves me with co2. I do not have not a spec of bba in the tank but I just can not get rid of gda. No animals in the tank except for a few snails, so no overfeeding. Matter of fact, I don't feed at all. I also do not think it is po4 since I dose on the upper end. I know some people dose 5ppm 3x a week without any issue and I am right around that area per wet's calculator. Also, dc is in the yellow range by the end of the day. Any thoughts?
Possible to have low CO2 without bba? Certainly. Every low-light, low-tech tank is an example. CO2 isn't necessary at 30 PAR, just helpful.
Low PO4 typically causes GSA, not GDA.
Your plants may be struggling a bit at this light level. For high-tech tanks, I find medium-low light (about 50 PAR) to be the most stable and algae free level.
Nitrates require more energy (in the form of light) for plants to utilize as a nitrogen source than ammonia from livestock. Which means they have less energy available for other things like growth, maintaining good health, and keeping algae down - however they manage to do it. They need more light to compensate for a high nitrate and nearly zero ammonia situation like yours. If not provided, GDA is indeed one possible outcome in my experience.
How long is your photo period, I found reducing mine by 2 hours cleared up my GDA and I had it really bad, now its barely noticeable. Although, now another algae has poped up in its place dealing with that right now.