Leaf damage and deformation - please help me diagnose!

Attached are the pictures of my young child's damaged leaves. The leaves are opposite one another - and the damage starts as a discoloration of the fan leaf and eventually becomes the necrotic, orange/yellow you see below.

Whatever it is seems to be spreading slowly to the leaves adjacent these two (on the same stem).

Temps have been fair to high (working on that) and I'm watering every 3-5 days.

I've begun using a 25% strength solution of Rhizotonic from Canna (awesome product for germing seeds, btw), but I don't think that's causing any nute burn.

Anyway, I'll let you guys tell ME. Thanks!

Pics didnt show up but make sure you check your PH and it also doesnt hurt to add cal mag to your feedings.

That looks isolated and the fresh growth looks lush and green. Here is a link to help you identify common problems.

Thanks for the quick response. I'd like to note that the problem is spreading, albeit slowly, to the leaves directly next to the ones that display the biggest issues.

Any other thoughts?

Thank you again for the help!

Make sure PH is not too far out of wack and get back to us if it isn't.