Fe to low, Quiet CO2 into Aquaclear

I have been adding Scholer Natural Gold liguid Fe and trace
element additive to my plant tank, the readings on my test
kit (Seachem) never get above .4mg/l which is I guess the
same as .4ppm. I've added the correct dose 4 or 5 days in a
row. Since my plants never received liquid Fe supplement
before, are they just using it all up? Should I keep adding
it until I reach .8? I don't use carbon in the filter or any
water additives.  Also does anyone know how to inject CO2
into an Aquaclear 300 quietly? It does a great job of gettin
g it into the water, but that sound of the bubbles getting
chopped up.. I hate it.  It is in my bedroom.
Thanks, Mike