I'm inclined to guess that he did blow out his tail. What has me believing it is not fin rot is the fact your water quality sounds good, with everything testing as 0 ppm. Secondly, he recently discovered his reflection so I'm sure he has been all worked up and flaring a lot more than usual. Turning down the filter and adding an almond leaf sounds like a great plan. Be on the look out for any changes in his condition.