water changes

I had a suggestion come back that the reason I was not detecting any 
iron in my 180 show tank is that I am changing too much water. Although
I don't think this explains everything, it did cause me to look at my
water changes again. I have a system setup so that all I have to do is 
open a value to drain the sump in the W/D (which is about 5 gal). The 
float in the sump then refills to the normal operating level. Given I 
have been doing this when I do feedings (twice a day) this turns out
to be 10gal a day or ~70gal a week. Being brain dead, I somehow got it
into my mind that this would be ~20% a week... not closer to 50% a week!
No wonder I have been going through so much Dupla ($$$). 

How much water do people usually change on a show tank with lots of plants ? 

Given that I now have a significantly lower level of DuplaPlant then I 
orignally thought how should I correct this? Do I just dump in enough 
for the full tank again (~35tablets)!?

Also, even if I have been changing too much water, I don't think that this
explains why a full dosing of ferro vit didn't show a reading on the Red Sea
iron test kit I have (remember, I do get a reading on my 30gal tank that I 
have not been changing water on).

Any thoughts ?