How often do you fertilize?
Maybe you've actually added too many nutrients. How did you determine a Magnesium deficiency, and then how much did you add to your plants?

How much sun / heat do the plants receive?

Josh

Look for broad Mites.
Get a magnifying glass.
Mites can find a Rocoto/Manzano plant from miles away.
They love them...

I don't know how to determine mag deficiency. I made an assumption based on research. As to too much fertilizer, the problem started when I began the seedlings on GH Flora 1-1-1.

The plants are under a Sun System VII 250mh/250hps for 18hrs/day and the ebb & flow pumps 4 times a day. The temp is a constant 70 degrees.

I checked for mites and found none.

The Flora series is a three part system (Grow,Micro,and Bloom ) using all three from seedling on. Their Flora Nova Series is a one part nutrient(7-4-10) Grow alone for the first three weeks then combine with Bloom for a week then just bloom. You need to change your water every 7-10 days GH has a feeding schedule online under resources. With hydroponics you have to follow what ever feeding for the specific manufacture nutrient you are using since they are not all complete or compatible.
If you are using coco for your medium cut the number of cycles or mix course perlite 50/50. Also when you say 1-1-1 are you talking NPK or ratio of Flora?

This post was edited by randy355 on Mon, Sep 22, 14 at 2:10

It is MAYBE just getting cold ?

My Manzano ( Rocoto's cousin ) is doing fine so far. I just fertilize it along with the rest of my peppers with 1/3 strength of 12-4-8. Now I am not fertilizing anymore.

Another reason for yellowing can be just too much water.