Dupla iron supplementation: a query

     Thanks for the comment in response  to my post on the APL concerning 
iron levels in Kent supplements. I will be responding to some of the 
comments over the next few days. I suspect George Booth is probably correct 
that not all the iron which is measurable in the water column is utilizable 
by plants.

     My recent pondering about iron supplementation with the Kent product 
led me to wonder about the rationale for the Dupla system of trace element 
(especially iron) supplementation. Unless I am mistaken, Dupla drops, with 
their 1180 ppm iron (information from George Booth), are added in such small 
volume that they contribute very little to the total iron concentration in 
the tank. Assuming .05 ml per drop and 4 litres / gallon, addition of a 
daily dose of 4 drops to 85 gallons (again, chosen from an example by 
George) would raise the iron concentration of the tank by only .0007 ppm [4 
x .05 x 1180 / (85 x 4 x 1000)]. Even a week*s, daily additions would 
contribute only .0049 ppm to an 85 gallon tank, i. e., only 0.5% of 0.1 ppm, 
considered by many a reasonable target concentration or iron. Thus, tanks 
which are run according to the Dupla system must derive most of their from 
products other than the drops. Information posted on the APL indicates that 
the principal other iron sources among the Dupla products are (1) DuplaPlant 
tablets and (2) laterite.

     But I do not understand the rationale for adding iron in both the 
tablets (with water changes) and daily drops. If the plants need the drops 
for the extremely small amount of iron they contribute, what is the purpose 
of the iron in the tablets? If I am not mistaken, information from previous 
APL posts indicates that both the drops and tablets contain chelating 
agents, so the iron in both products is apparently chelated. Is this 
correct? Dupla says that the purpose of the drops is to add materials which 
are either unstable or sufficiently toxic that they cannot be dosed to last 
more than a day or so at a time. The chemical analysis of the drops 
suggested that iron-EDTA is the principal, if not the only, ingredient. Is 
it the iron which is unstable or toxic at higher doses? (As noted above, the 
quantity of iron added with the drops is way below the levels maintained in 
most tanks, so clearly the iron in the drops is not toxic.)

     Thus, my basic question is, why is iron added both with the tablets and 
with the daily drops, especially when the quantity of iron added with the 
drops is so miniscule?

     Laterite is the other, principal iron source. Does anyone know how much 
of the iron in laterite escapes to the water column? Any data on iron 
concentrations in aquarium water with laterite which has not received 
another source of iron?

     I am probably missing something here, somewhere. Can anyone enlighten 

Richard Denney.