RE: iron in Kent supplements

I awakend this morning realizing that I had made an arithmetic error in my 
post from yesterday (Nov. 20th) concerning iron in Kent freshwater 
supplements. Despite the error (explained below), my calculations still 
suggest that regular, weekly use of Kent supplements according to the 
instructions on the bottle, which recommend a weekly addition based on the 
total volume of the tank, would be expected to lead to a steady increase in 
iron concentration in the tank well beyond the 0.1 ppm which is commonly 
recommended for good plant growth with minimal stimulation of algae. 
Overdosing can be avoided by adding a low dose of Kent supplement based on 
the volume of water exchanged, _not_ on the total volume of the tank (see 3 
below). Daily addition of very small doses can then be used to maintain the 
concentration near the preferred level.

For those who want details:

Yesterday, I said:

> the Kent supplement at the dosages recommended on the bottle will almost 
certainly result in >an initial iron concentration in the water column at 
least 14-fold higher (and as much as 42-fold >higher) than that which 
experts on the APL recommend for plant tanks (.1 ppm).

1. This statement is not true, since my estimate of final iron concentration 
was 10-fold too high. (The formula was right, the arithmetic was wrong!) The 
estimate was based on Kent's recommendation to use 1 to 3 tsp per 30 gal. 
Kent supplement has an estimated 2000 ppm iron (my measurement). Therefore, 
the final concentration of iron will be approximately (assuming 4 
litres/gallon, and 4.8 ml / tsp):

     For 1 tsp: 2000 ppm x 4.8 ml / 120,000 ml = .08 ppm (not 0.8 ppm as 
stated yesterday).
     For 3 tsp: 2000 ppm x 4.8 ml x 3 / 120,000 ml = 0.24 ppm (not 2.4 as 
stated yesterday).

Thus, adding the lowest, recommended dose of Kent to the tank should _not_ 
result in an iron concentration greater than .1 ppm, though the highest dose 
(3 tsp/30 gal), would.

2. This error does not affect yesterday's argument that iron would be 
expected to build up over time if the supplement is used according to Kent's 
instructions. The Kent supplement bottle recommends adding the same dose (1 
 - 3 tsp/30 gal of tank volume) every week. However, the actual utilization 
of iron by a planted tank appears to be much less than this, based on a 
representative tank mentioned in the APL by George Booth, which required 4 
Dupla drops (iron equivalent to 2.4 drops of Kent supplement) each day to 
maintain a preexisting iron concentration of 0.1 ppm. Assuming 0.05 ml/drop, 
this suggests that only .175 tsp/week of Kent supplement, not the 2.8 
tsp/week recommended as a minimum by Kent, should sustain the iron needs of 
his tank. Daily addition of 4 Dupla drops (or 2.4 Kent drops) would be 
expected to change the iron concentration of the 85 gallon tank by only 
0.0007 ppm, i. e., hardly at all. One implication is that if added on a 
weekly basis, most of the iron present on day one should still be around on 
day 7 days. Consistent with all of this, I have on two occasions detected no 
change in iron concentration in my heavily planted 75 gallon tank over a 
three day period (no daily addition of iron supplement, no water change; 
initial and final iron concentration, about 0.15 ppm).

These arguments indicate that additional of 1 tsp Kent supplement per 30 gal 
to iron free water on days 1 and 8 would nearly double the iron 
concentration from the original 0.08 ppm (actual estimate, 0.157 ppm.) 
Although water exchanges with iron-free water could reduce this, it would 
take a 96% change prior to the second addition to restore the iron to the 
initial 0.08 ppm.

All of these estimates are based on the assumption that iron which is 
measurable in the water column (even that added many days before) remains 
utilizable by plants.

3. Recommendation. Simply dosing exchanged water with Kent supplements based 
on the exchanged water volume (rather than the entire volume of the tank as 
recommended by Kent) should eliminate the accumulation of iron in the tank. 
The only change in iron concentration then would be due to that which 
disappears (utilization by plants, binding to substrate, precipitation, 
etc.). The small loss of iron can be replaced by additional, small, daily 
doses (1 drop per 35 gallons of tank volume is a reasonable place to start). 
 The daily dose should be adjusted based on assay of the iron concentration 
over reasonably long periods of (weeks or months).

4. Users of other chelated iron products which recommend addition weekly or 
with water changes might want to consider their own practices and the 
recommendation by the respective manufacturers. For example, Tetra Floral 
Pride claims to contain 0.15% (1500 ppm) chelated iron. Tetra recommends 
that it be used at an initial dose of 10 ml per 10 gallons. This would be 
expected to yield a final iron concentration of approximately 10 ml x 1500 
ppm / 40,000 ml = .375 ppm chelated iron, which seems high. Subsequent 
additions are suggested to be at half the initial dose, or 10 ml per 20 
gallons at each water change. If these subsequent doses are based on the 
total tank volume, the iron concentration will almost certainly increase 
with time despite reasonable water changes.

Hope this discussion is of use to somebody.
Sorry about the error yesterday.

Richard Denney