Is This The Classic Magnesium Def?

Zinc deficiencies are very, very rare. If those are lower, older leaves (and it looks like they are) then I'd say yes, that may be the start of a Mg-. An easy, quick way to tell is to do a foliar application of 1/4tsp Epsom salt to a gallon of water. Don't forget to use a sticker-spreader, liquid dish soap or similar.

I was hoping to get an answer from you @Seamaiden. I have been foliar spraying one of them with Epsom salts 1-2x a day for 3 days now to see if it would get better and it's still showing these same symptoms, I will keep doing that.

The Epsom salt that I'm using is "bath" Epsom, should I get a different one from the pharmacy, one that I myself can drink?

As for the light green colour on top of most of the plants that you see clearly on the 3rd and 4th picture - do you think that also, is Mg def?

Ok, if it's not clearing up within hours, then that's a diagnostic telling you this is NOT a Mg-. I use the same kind of Epsoms, MgSO4.

The light green color of the new growth looks like a normally growing, but also very vigorously growing plant. They'll do that when they're growing fast, it's to do with translocation of N which is the start of photosynthesis or something like that.

Hmm... I hesitate here to suggest ranging pH, in most cases that is my usual suggestion. They look pretty damned happy otherwise, just that thing. Have you tried slightly increasing the ferts?

They have been growing very fast so that explains it and yes the last feeding they got, which was 2 days ago was of a higher EC than these last 2 weeks.