PH fluctuation

4 5 gallon buckets with a 15 gallon res.
6000gph pump circulating the water.
PPM was set at around 900 and PH 6.0

What has been happening is my PH has been dropping to the 5.5 or lower range daily. I don't know if it is just coincidence, but it started happening when I switched to 12/12 light cycles. Prior to this happening, it would stay very steadily at 6.0 ph. What also appeared to have happened is the PPMs went up. When they used to read 850 to 900ppm they started to read 1000ppm.

Is this something I should be worried about? There is a small amount of algae growth, but I now have it completely blocked off from receiving any more light.

I did just finish doing a partial system water change. I figured it would be a good idea since I was having such fluctuations, and it would also allow me to start with the bloom nutes.

Thanks in advance for any help.

In my system that is quite normal during large growth spells. I see my largest PH drop from week 2-4 in flower why they are getting tall, then it calms down again till about week 6 when I start to notice the bud filling out. I think it happens when the plants are in big development phases, all of my harvest have done the same thing, I even posted the same question 6 months ago.... Seems normal, I get good yeild, color, and high.