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Re: iron levels

"Hello, Could someone tell what the iron level should be in a planted tank?
Thank-you, Ray"

Well, I have a very good iron test kit and I was consistently measuring
about 0.03 mg/L Fe in three 20 gallon tanks. I was also having problems of
very slow growth despite good light levels and other nutrients. If I missed
a trace element dose it really showed. Yellow margins or yellow spots on
the leaves and pale leaves with green veins. This was evident in both
rooted plants and non-rooted plants.

The substrate for these tanks, now about a year and a half old, is soil
based and uses Turface. After reading Ecology of the Planted Aquarium ( D.
Walstad, Echinodorus, 1999) I was reasonably sure I had plenty of iron in
the substrate. I did some long term tests on the soil and found that the
iron content remained at about 0.04 mg/L. As an added measure I sprinkled a
pinch of iron filings on the bottom glass before adding the substrate. I
have used this method several times in the past and can see the filings
rusting on the bottom if I can observe the bottom of the tank. This being
the most anaerobic area in the tank, I thought it would provide iron in the
correct state available to the plants in a low but continuous
concentration. I did not want to overdose. Both the soil test and the
current conditions of the aquarium water show consistency in the iron
level: about 0.04 mg/L.

So, even though I thought I had a sufficient supply of iron in the
substrate, both rooted and non-rooted plants showed the same marked
deficiency problems. I accidently figured out that I must have too little
iron after overdosing one day with trace elements and both types of plants
responded favorably. I started adding twice and then three times the amount
of trace elements and measured no change in the iron content of the water.
So, the plants were sucking it up as fast as I put it in. After starting
iron supplementation in the water both types of plants immediately showed
improvement. I guess the amount of iron in the soil was never enough or
that it has been used up after only one year. I now maintain "more" iron in
the water column. It varies between 0.6 and 1.3 mg/L All I can say is that
0.03 - 0.04 mg/L is not enough for the light level I am using; 2
watts/gallon. I am now adding half the recommended dosage after a water
change and that is still providing the higher levels that I am measuring.
It doesn't take much.

 --- Eric