Transparent spots on leaves

Hi,

I've got some strange transparent spots on the leaves of my planta.

Photo: see at the end of post (I cant post imgur photo link because I'm new and have no "Likes").

I can't find any bugs or mites. It looks like the chlorophill cells are dying and I don't know why.

The spots are bigger from day to day. (They were only on 2 of may plants on the first day, 2x size on second day, and on the third day they appeared on some leaves of about every second plant of mine)

GROW CONDITIONS:
Indoor, hydroponics
Temp: 26°C (79°F); Hum: 59%
Light: Phytolite NX2 400 LED; 18/6
Plant size: 20 cm (8 inch)
3 week old plants, vegetation phase
Seed: Kera critical
ph: 6.0, EC: 1.2, RO-filtered water with "Advanced hydroponics of holland" nutrients

What you think, what causes it and what can I do for my plants?

Thank you for your answer!

Regards,
Peter


A spot with 60x microscope:

What colour are the stems of the leaves pal ? Deffo looks a bit like phosphorus deficiency to me, what ph is your water and what are your temps ?

Only just seen your temps and that sorry mate, I'd deffo say it's phosphorus deficiency or at least a nutrient problem but I couldn't tell you what's causing it mate

This may help