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FE as a limiting nutrient

I have always remembered being somewhat inspired reading the original Conlin
and Sears paper about limiting nutrients and algae. I always thought it was
cool to be able to measure nutrient levels, find one nutrient that was
deficient, add the deficient nutrient, and then observe another nutrient
level decrease as growth of the plants increased because of a balance of
nutrients consumed by the plants. However, I never really was able to
observe it firsthand. If I added iron to one of my tanks, the iron levels
would remain the same after a week. The same with nitrate. The levels would
not decrease significantly over time. Growth of the plants was OK with the
strong lights and Co2, but often so also was the algae.

I started a new 125 gallon tank in May. At the same time I took down my 75
gallon tank to move it to another location. So I ended up setting up two
fairly large tanks from scratch. Until last week the plants in both tanks
were growing at a slow rate, and both tanks had a persistent algae problem
that was inherited from the old plants originally from the 75 gallon tank. I
was adding the recommended amounts of nutrients to both tanks per a
discussion with Tom Barr. Since these nutrient amounts were considerably
more than I had used previously, I hadn't checked nutrient levels in a few
weeks. Last week when I checked the iron in both tanks it was zero per my
Hach TPTZ test kit. So I doubled up on the iron that day. The next day, same
thing: zero Fe, and added another shock of iron. This has continued all
week. Although I just purchased a 2 liter bottle of Flourish, I was thinking
that at this rate it would be gone in a short time, so I started using some
Flourish plus Iron I had since it has a lot more concentration of Fe in it.
I have gotten to the point where if I dose the 125 gallon tank with 1 capful
of Flourish plus Fe (.13 ppm) it will go down to about .03 ppm in 12 hours.
That comes out to 1.8 ppm Fe per week for the 125 gallon tank!, (based on
100 gallons of water)...equivalent to 35 capfuls (5 ml each) of Flourish per

At first I thought that something else other than my plants was eating the
iron. But one thing did happen last week that makes me think not: the plants
grew between 75 to 100 percent in that time. Also the algae problem in the
125 gal. tank has greatly diminished (just about disappeared), and the water
is now crystal clear. I was using a Vortex XL diatom filter on the tank
24-7. The filter media would turn dark green. I would get dark green blue
algae on the plant leaves that wouldn't come off. I would also get a green
algae on the tank glass. Since this tank has local biotope plants in it I
have been not only pruning the plants of algae infected leaves but also
replacing plants, when possible, with fresh algae-free ones from the
ditches. I think the combination of constant and severe algae removal and
nutrient additions has really reduced the algae problems in this tank, to
the point where I can start getting it ready for the AGA contest.
The 75 gallon tank is not as fortunate. I do have the growth similar to the
125 but there is still new algae growth. The difference is that I can't
remove all the algae from the plants in this tank because they are my older
plants from the previous set up. They had algae on them before, and since it
doesn't rub off, its hard to prune most of the leaves from the plants. These
are my non local plants, the exotic ones that I had purchased in the past. I
can't afford to replace them if I throw them out, like with the 125 gallon
tank and the local plants.

One possibility as to why the nitrates have not gone down is that I have
about sixty local sailfin mollies of all sizes in the 125 gal tank. Perhaps
they are giving the plants the needed nitrate levels, and preventing the
nitrates from being exhausted. I was afraid all the fishfood would provide
too much phosphate, but with the decrease in algae I suspect not.

Am I doing anything wrong? Anyone have a cheap way to supplement just iron
(hopefully a chelated form)?

I am considerig throwing out the exotic plants in the 75 gallon tank and
replacing them with Bacopa monneri, which happens to be only locally
available (from the ditches)aquatic plant at this time (and the Florida
Invasive plant site considers it a terrestrial). I do like the java ferns
and Telanthera but I think that you've got to be quite ruthless at the algae
removal if you don't want it to come back.

Steve Pituch