Yellowing

Is that just on one leaf? Post a pic of the whole plant along with your soil pH and PPMs

We would need a lot more information in order to properly diagnose. Here's a template which will give us just about all the info needed: What strain is it?

Is it in Veg or Flower stage?

If in Veg, for how long?

If in Flower stage, For how long?

Indoor or outdoor?

Soil, Soilless or Hydro?

If soil/soilless, what is in your mix?

If soil/soilless, what size pot?

Size of grow space (LxWxH)?

Make and model of grow light?

Temperature in the environment?

Humidity in the environment?

PH of medium or reservoir?

PPM of medium or reservoir?

Any Pests?

How often are you watering?

Type and strength of nutrients used?

Been watering once a week with mag con.

maybe overwatering , what's your feeding & watering schedule ?

looks like potassium deficiency.