Agreed, looks like a pleco got it. It is a sword, so trim the old leaves off. Got a plecos, remove the offender. Plecos will hammer all the leaves, maybe the newest leaves will not be gnawed, but not long after they unfurl.............
Swords grow like mad. No fert issues I can see, the plants are tough as nails. ADA AS covers most things and then water as well.
Light is fine.
Gh booster can be and should be added.
Tap water change change dramatically in some locations during the spring thaw (or freeze), KH's can move around a great deal and the Ca and Mg can drop if you do not add GH booster.......
They also might add more Chlorine etc..........so if you added little dechlorinator........there's all sort of variation due to seasons and other factors with tap, but generally, if you add the GH, and the KH does not drop below say 0.5 or so........you should be fine. Gh is more an issue with this than KH. Check for any CO2 leaks, stuck valves etc.