🚀 go-pugleaf

I have been keeping plants in my two tanks for quite a while, and although they
grow well, they tend to stay smaller than what they were when I originally
bought them.  As an example, I bought a giant amazon (A. bleheri) 6 months ago
with leaves (plus stems) that reached about 15 in.  Now, all new leaves are
only half that size (most of the difference is in the stem).  The same happens
with my cryptos and hygrophilas.  
I add weekly iron and trace elements, and have laterite in the bottom gravel. 
I use 3 40W Triton bulbs on the 70G tank and a reverse-flow Eheim undergravel
filter.  I have a relatively small amount of
hair algae among my floating plants.
My question is: could this lack of "big" growth be the result of CO2
deficiency?  If so, what would be an acceptable level and how is it reachable
(preferably without having to buy a $$$ diffuser...)?
Any thoughts on this would be greatly appreciated,

John Opsomer
opsomer@orie.cornell.edu