This is not a CO2 (or Mg deficiency). It was either damage during shipping (too cold/hot/wet or a lack of light. You can tell because the damage is extensive - leaves all died and became gelatinous which is not the sign of a nutrient deficiency they also show uniform damage. Deficiencies are not quite as aggressive as this, nor do they affect so much of the plant.
CO2 deficiency causes no physical harm to the plant over the short term. The only change is plants stop growing, but all tissue is healthy. Some plants will show biogenic decalcification on their leaves (white precipitate) but no physical damage.
Magnesium deficiency is indeed in the old growth, but it is very distinctive, dark green veins will stand out on pale and chlorotic leaf tissue between the veins.
happi has a point, there are lots of deficiencies that affect old growth and we can't suspect them all just because the damage takes place on old growth. Looking at the pattern of damage helps rule out many of these issues.
is there any way to test for Mg? and what's the desired range?
While you don't need to, you can find out Mg levels by testing GH, measuring the calcium and subtracting the calcium from the GH reading (converted to ppm from degrees). GH is basically Ca+Mg. Appropriate levels somewhat depend on calcium, you don't want to have extremely high levels of magnesium and low levels of calcium as that can cause problems. A GH of about 6 is usually sufficient to grow most plants (and a 4:1 ratio of Ca:Mg seems to be the usual ratio between the two nutrients in most naturally occurring ground water).