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nutrient deficiency?


I have a tank that's been growing so well for the last several months that
I stopped paying a lot of attention to it.  Now I've got some plants
showing a few odd problems.

Late last winter I put a few stems of Egeria densa into the tank to shade
lower-growing plants.  It grew like Egeria densa is supposed to grow - it
was a weed.  Then about three weeks ago it suddenly started growing very
slowly. The growing tips became nearly white and the internodal spacing
near the tips was very small.  To the touch these tips were firm and a
little brittle.  Growth of most tips eventually stopped and the tips died.
The plants had fulfilled their purpose, so when the problem came up I
removed them from the tank.

I think this may be an iron deficiency, but I'm not sure.  I add iron to
this tank with bits of ferrous gluconate pushed into the substrate and
since the Egeria was intended to be a temporary tank inhabitant I didn't
bother to add any iron for it.

Can anyone confirm that this is an iron deficiency, or provide an
alternate explanation for the problem?  If it does prove to be an iron
deficiency then I think that might argue for Egeria densa serving as an
indicator plant for iron.

The second problem came on at about the same time, or maybe a little
later.  The older leaves of an Aponogeton longiloplumis(?) started turning
dark purplish brown.  The affected leaves also got rather thin and were
easily damaged.  The leaves still produce bubbles late in the day, so I
know they're alive and functioning.  Also at about the same time the plant
put up a flower stalk (with a bud that looked oddly more like a
Cryptocoryne bud than any Aponogeton bud I've ever seen), but the bud
stopped at about mid depth in the tank rather than breaking the surface.
I noticed it more than a week ago and it still hasn't opened.

The discoloration in the Apon. leaves appears on the old growth, while the
problem in the Egeria was on new growth, so I doubt these are different
expressions of the same problem.  Could the discoloration be natural for
this plant?  Phosphorus shortage is supposed to sometimes cause a leaf
discoloration like this in some plants, but it seems odd that the tank
might be low in phosphorus.  I can't use phosphorus test kits because of
the high silica in my tap water.

Any ideas, or experience with similar problems?

Roger Miller
in Albuquerque, where recent rains have the desert blooming and I seem to
be (achoooo!) allergic to something out there.