I have no idea. I have two guesses; I introduced some plant clippings from a friend's tank into both of my tanks and I have recently stopped running the filter in both of my tanks. My plants grow better when I leave the filter off. I think it might have been both of those things.
I do have a baffle on both my filters but I need to invest in some foam to slow the water flow even more. I should have quarantined the plants but my friend has a healthy, thriving, planted tank. I just added them without thinking. I added them a week ago though...
I test the water in both of my tanks weekly, and do a 50% water change. They are both cycled and the levels have been good. The Ph has fluctuated a lot over the last month but I have no idea as to why and it has leveled off.