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
For 3 tsp: 2000 ppm x 4.8 ml x 3 / 120,000 ml = 0.24 ppm (not 2.4 as
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.