Quote Originally Posted by UncleChester View Post
Too much water can cause leaves to turn yellow, often the lower ones go first if this is the problem and they can eventually rot off. Is the substrate constantly wet or soggy? Damp is usually ok, but wet can be bad depending on the plant. Too little fertilizer can look similar, but if your plants are still growing this is likely not the issue.
The ABG Mix is damp but not soaking and I do keep the tanks damper to maintain higher humidity. I've lost a few bromeliads to rot!

Typical symptom of too little light is the plant will stretch and and get 'leggy' trying to get closer to it, not yellowing of the leaves in my experience. Some variegated varieties may tend to a plain green under low light, but not head towards yellow.
I know the 40 vertical suffers from lower light at the bottom but that is not where the leaves on my lance leaf ficus are turning yellow. Just so many variables on this issue.