If not nutrient burn

could anyone tell me what it is?

water pH might be too high , try adjusting to 5.5-6.0

one leaf isn't enough to identify anything

picture of the entire plant please

The rest of the leaf is fine. I don't think it's much.

Use one or more of the online cannabis leaf nutrient deficiency charts to diagnose it until you get a understanding of what different deficiencies look like.
To me it looks like a cal mag or potassium deficiency to be sure do a google search and look at a few charts.

htownchoppa1 said:

could anyone tell me what it is?

Click to expand...

it's a marijuana leaf

Like stated it's hard to tell anything from one leaf but as tty says, if it's only one then probably nothing. The first thing and easiest if a flooded root zone is check the roots and go from there.