strange rust patches

[Gangabiss]

I've looked all over their website and it mentions nothing of the ingredients...
I have actually just noticed today that some plants are developing slight yellowing between the veins on the lower leaves.

A couple of plants seem to be stretching a lot more than the others which is quite strange as the rest are all very nice and bushy. These seem to be the ones worst affected by these patches.

I'm thinking of giving them 1/2 strength nutes in 4 days and then repotting them a few days after.

 

[sproutco]

Here is a full blown case of magnesium deficiency if you want to compare your plants to this: http://www.icmag.com/ic/showthread.php?t=37463