That is pretty stable, but with nutrients as low as yours the threshold for change is also much lower. When the corals are relying solely on light for energy, any change to light intensity or alkalinity can push them over the edge. Your corals were likely suffering from malnutrition and then by adding light, they got a bit of sunstroke and since already weakened gave up.
We spend a ton of time and energy trying to maintain alk,cal, mag balance but we don't pay enough attention to nutrient balance, but if you ask around most if not all of the long term successful tanks owners will tell you they always test the same whatever the balance is for that tank and the numbers usually end up, no3=10 to20 ppm, po4= .03 to .1
The ulns like Zeovite are riding a knife edge but are also supplementing a ton of nutrients and aminos to feed the corals. The rest of us have no3 and po4.