What kinda of deficiency is this?

Is this a calcium deficiency or magnesium? And what should I do to fix the issue

get some cal mag for her, treat a little every so offten if its soil, but if its coco, wait for a coco grower to answer your questionj.
if you tell us a bit about your grow , you will get a better answer
the newer leaves look ok, so it may be a past problem, keep an eye on her.
has this just started happening?

Could this occur from accidentally letting some of the water get on the leaves when I was feeding them?

could be, need a closer picture to see properly, but you usualy get a dark discolouring where the water touched the leaf

no, thats not from water splash.
has it just started, or was it as the leaf grew it showed up?
its allways best to use cal mag on your plants, they controle the growth

did it show up as the leaves grew, or has it just started happening

As the leaf grew it showed up I haven’t used any cal mag on the plants yet

WHATS THE PH OF THE WATER YOU FEED THEM WITH,oops shouting again. and what type of water you feeding them, tap, well, bottled???

Bottle water and my ph meter was off so the ph may not have been correct but the last few feedings the ph been 6.1

is that bagged soil your growing in?

personaly, i would get some cal mag anyway. i personaly wouldent worry about the brown bits, unless it starts to get worse. leaves do all sorts of stuff

Ok thanks for the feedback bro