The Case of the Disappearing Iron

I'm stumped. In an effort to get the pH of my water down, (it was
consistently sitting at 8.5 and higher) I broke the entire tank down, bought
new gravel, which I treated with Muriatic acid to make sure there were no
alkaline buffers present, and then reassembled the tank. ( I also added a
DIY undergravel heater using PVC pipe, a pump and regular water heater
:-). Before I put the plants back in the tank I soaked them for two minutes
in a mild bleach solution. (the vallisnerias did not like this at at all by
the way). The pH is now stable at 7.2. The water is liquid rock at about 450
GH and 80 KH. Thing is, I cannot keep the Iron level up. I routinely add
Flourish, Aquatic Plant Stimulant and now straight chelated Iron.
Regardless, within a couple of hours the iron level falls to unmeasurable
levels. Is it possible that the compounds that account for the high general
hardness are "instantly" binding the added Fe?

I do not use any carbon in my filters (two magnum 350s). One filter has
plain gravel in it from the original tank and the other filter is a
so-called "micron" filter that provides just mechanical filtration of small
particles. The only other thing is that I am using DIY CO2 injection and
keeping the CO2 at the optimum range as indicated by an in-tank Dupla "Bell".

Any ideas?
Justin Healy in Sunny, HOT Savannah, Georgia