What is wrong with my plant

[Ossi Ossi]

Hi
This has started to show up on some leaves
Any ideas where to start
Cheers

 

[improvise]

Welcome. Cannabis Plant and Pest Problem Solver - Pictorial . regards

 

[Emilya Green]

I will start off by saying that it looks like a magnesium deficiency. Is this an LED grow or one using filtered water? Both of those conditions would make this more common.