Random thought and I could be way off base, but I had a similar problem (I am hoping is resolved) and I discovered that even though I was testing po4 the same time each time and it was around 0.04 ppm, my tank had a demand/turnover of about 0.07 ppm of phos per day and this was stressing/starving my acros. ( I realized this by testing it multiple times throughout the day to see if it bottomed out and it was doing that during the brightest light period when they take up the most stuff)