Is this a calcium deficiency?

Had a couple of Critical Kush clones in 1 gallon Down To Earth Pro Organic mix which I supplemented with 3/4 slow acting vegan fertilizer and 1/4 c kelp meal. They've been watered twice (lightly, 12 ounces water) with filtered tap water, I'm brewing the Earth Juice/worm casting tea to give them the first feeding tomorrow to kick off week 4.

I've noticed some brown spots developing, and one leaf has some slight yellowing. I see no signs of bugs, I've done a couple of neem oil sprayings just preventative.

From some diagnosis web sites, the two options that seem to match are Calcium or Manganese deficiency. I think Calcium is the more likely option. If anyone can conform/deny/offer other opinions, that would be cool 'n all. If it is what I think, I'll add some cal-mag to the final brew tomorrow.

Probably you right. Cal-mag will be good decission for plants.

I _should_ be getting cal in my tap water though. I use a Berkey water purifier, it's not supposed to remove calcium and magnesium. Since I should already be getting it, I don't want to create a lockout situation by having too much. And I'm at a loss to think it's any sort of over-fertilizing, since I've only added slow acting amendments to the soil, so any sort of flushing seems pointless to me. I've only had them a week, so this showed up rather quickly, but it's not just the old growth, I am seeing signs in the newer growth too.