vermiculite, iron, and potassium

I recently read the book "Hydroponics for the Home Gardener."  It had some 
interesting information that I have never heard about in my readings of 
aquatic plants.

For people using vermiculite:
"the vermiculite you use should have as neutral a pH as possible and no 
toxic amounts of boron or flourine." 

Regarding trace elements:
"Iron becomes less available to the plant as the hardness of the water 

"Under very humid conditions, where the light level is lower, you plant will 
require more potassium -- as much as twice the normal amount.  This is 
because photosynthesis is more difficult with a lower light level and 
potassium is necessary for photosynthesis."

I know hydroponics is drastically different from an aquarium, but does 
anyone have any thoughts on whether the above applies to aquariums?

-- Darrell