Hey,
Just by the pictures, I'm guessing your ph is a little low. This will cause calcium, magnesium, zinc, and iron to be locked out. Make sure you're measuring the ph of your runoff water. Pm me if you have questions.
Cheers,
It could be a lot of things, without any info on what they are being fed/how much/how often/ temp/ humidity/ etc. its harder to make an accurate diagnosis.