disintegrating

Hi all, Long time lurker here, finally joined up! I have 3 tanks (2 X tropical and one cold water). One of my tropical tanks is semi-planted (mixture of live and silk plants - seems to keep my Betta happy!) But recently one of my plants has started to "disintegrate" (but only on one leaf so...

I have a Juwel Rio 125 litre (T5 lighting, I think; def not LED), “pimped” internal Jewel filter with 2kg of biogravel. Sand substrate over Tropica aquarium soil. . pH 7 - 7.5. Tap water source is hard (London) - DH14. Amonia, nitrate and nitrite levels all fine. Pond snails. No fish yet. The...