Calathea musaica network – ragged pale leaves

Hi

I was gifted a calathea musaica network plant on 22nd July (sent from an online plant shop).

When it arrived a few of its leaves had yellow/brown marks on them, almost like cigarette burns. Since then, the marks may have grown slightly (although it’s hard to say for sure because I didn’t take photos at the start), but also some of the new leaves have turned out paler than the main growth, and ragged, almost as if something has been eating them.

It doesn’t seem quite right so I’m not sure if there’s something particular I should be doing to help it. It’s just back from a south facing window so hopefully not getting too much direct light, I’ve only watered it a few times so far (most recently with distilled water as I had a feeling it might be disagreeing with the tap water).

Other than a few bits of dirt I can’t see anything that looks like bugs on the leaves. I did spot a small fly at one point that I thought might be a fungus gnat, but even if it was a gnat I haven’t read anywhere that they would cause the holes in leaves that seem to be showing up.

Any ideas or advice much appreciated… thanks

Tam 😊

You can see a close up of a growing leaf to the right, where it already looks like there's holes developing in it even before it's unfurled (also shows a pretty bad small leaf to the left which I guess may just need cutting out): 
Placement just back from the window:

The yellow/brownish marks:
The 'ragged' leaves - holes in and around the edges, new growth slightly paler than the original leaves: