It looks like potassium deficiency. What soil are you using, what are you feeding? Excessive magnesium or calcium can interfere with K. Sometimes people overdo it with "calmag." Are you using that? Excess P can do that to.
Are you able to check the runoff ph? (I don't think that's an accurate measure of ph. But, if the soil was very acidic, it might show that).
(Is that plant sharing a container with another plant?).
I would think you're watering too frequently (every 2 days). The soil doesn't look that light and airy enough to dry that quickly. The soil should be exceptionally dry when you water. If you hold the soil too wet it can make it more acidic which would make K unavailable. But, the plants don't look overwatered.
If you could measure your runoff ph it might say something. (When you water, do you get runoff? Or, are you just watering a little every 2 days?).
I do wait to water when soil is dry. I probably do need to give more water. Today i gave about half gallon each. I use a meter that checks moisture and i stick finger in it. I will wait till they are really dry next time. Nutrients are the cal pro AandB
I'm thinking light burn it looks very similar to K deficiency. K deficiency in veg is very rare especially so early in veg. The plants also look otherwise healthy so I don't think there is a nutrient issue. Also it appears to be only the upper leaves. K usually not always starts in the middle or lower portion of the plant.
I would do as @az2000 suggested before adding anything. It's highly unlikely there is not enough potassium in the nutes/soil and far more likely to be the availability due to nutrient build up, ph or other.