Have you made any changes in anything else (light, CO2, ferts, flow - ANYTHING)? Have your plants changed at all in terms of growth or health?
Pearling does not mean your plants aren't healthy. If your oxygen levels are at saturation, the O2 released by the plants can't be absorbed and you see the pearling. However, if your O2 is below saturation, the O2 will be absorbed before it can be observed as bubbles on a leaf. Higher consumption of O2 by fish and/or higher gas exchange at the surface from good rippling can reduce that saturation. I get very little pearling and only occasionally.
Of course, deteriorating plant health can also reduce it which is why I ask about any changes in plant health.