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nutrient-deficiency symptoms related to lighting?

This is going to be a little long...

Last March I posted a short description of an ongoing problem in one of my
tanks.  Briefly, the leaves of most plants developed lesions which
gradually expanded until the leaves become corroded and disappeared or
dropped off the plant.  Young leaves are largely clear of damage, while
older leaves are badly damaged.  A Bolbitis and Cryptocoryne balansae
didn't show the lesions, but the plants did not develop or maintain good
leaf growth.  The growth rates and photosynthesis rates (judging from the
O2 bubbles) of most plants remained high despite the damage.  These
problems started around 2 years ago and have gradually worsened. 

In hygrophilla corymbosa, Echinodorus bleheri (?) and java fern the damage
was most serious - in fact almost limited to - areas of the leaves that
were exposed to light. 

These symptoms might be caused by deficiency of a mobile nutrient -
particular potassium, but possibly magnesium as well.  I followed the
nutrient-deficienty angle because nutrient deficiencies seem to be
commonly diagnosed and shelved the less likely relationship to the

Nutrient deficiency at first didn't seem like a very likely cause because
my other tanks (using the same tap water) didn't show the same symptoms. 
I reasoned that the high growth rate in this tank (due to higher CO2
levels and brighter lighting) created a high nutrient demand that didn't 
occur in other tanks with lower growth rates.

The problem is in a 55 gallon tank that has been set up and growing plants
for about 8 years.  The substrate is coarse sand and gravel with
accumulated mulm.  There is an old and unused UGF under the substrate. 
The problem developed initially when there was 60 watts of light on the
tank.  I increased that to 160 watts with 3 Philips Ultralumes and a
Philips Advantage X, all at 5000K.  The increase in wattage didn't have an
obvious or immediate effect on the problem - but the growth rates jumped
and the problem gradually became more evident.  The tank gets yeast
generated CO2, weekly 9-gallon water changes and fertilization with Tetra
Flora Pride.  I've also used Kent freshwater plant supplement. 

The concentrations of both potassium and magnesium were fairly low in my
tap water - 2 mg/l of potassium and 1 mg/l of magnesium, according to the
water utility. 
So I went about changing things - lots of things.  I started dosing with
potassium (KCl) and magnesium (epson salt) to maintain concentrations at
10 and 5 mg/l, respectively.  It then turned out that my normal dosage of
Flora Pride should have created higher potassium levels than I was
shooting for, so I stopped adding the extra potassium. I started using RO
water just in case calcium interfered with use or uptake of either
potassium or magnesium, and adjusted the specific conductance down to
about 350 uS/cm - about average tap water but better than my tap water. 

Nothing changed.

I experimented with a potted H. corymbosa, using kitty litter mixed with
gravel in a pot to see if the increased CEC (relative to my gravel
substrate) would increase uptake of potassium or magnesium enough to fix
the problem.  The potted plant developed the same symptoms shown by the
other plants. 

I removed the over-the-back power filter from the tank (expecting the
decreased competition for ammonium would lower stress on the plants). 
This had no noticeable effect.  I also connected vinyl tubing from caps on
the unused UGF lifts to a venturi on a power head.  The idea was to
generate slow circulation in the substrate and "recharge" the exchange
sites in the substrate.  I waited two weeks and there may have been a
subtle change, but no big deal. 

That got me to the end of August without improvements, so I went for a
change in lighting.  I replaced one of the Ultralume lights and the
Advantage X with two Sylvania Growlux Wide Spectrum bulbs.  There was a
big drop in the light level.  The change in lights created richer brown
colors but didn't really change much else, color-wise.  The plants may
"bubble" less than they did and the growth rate may be a little lower. 

I saw changes within a week.  Now its about a month later.  The H. 
corymbosa that previously could only maintain 2 pairs of healthy leaves
now have more than six pairs of leaves.  Val. americana and C.  balansae
were stunted and have doubled their height - the val is starting to lay
out on the water surface and the balansae leaves are at 14 inches and
growing.  New leaves on the java fern used to show damage before they were
fully developed; the leaves that started growing after the lighting change
are reaching full development now and they're completely undamaged.  The
stunted E. bleheri (?) has responded by developing 3 new leaves, each
larger than the previous one.  Only the anubia barteri nana and the
bolbitis (both stunted) are holding out. 

So this *appears* to have been a lighting problem rather than a nutrient 
problem, but it leaves me with a bunch of questions:

1) I searched the archive and found that lots of us use 5000K ultralumes  
(5000K Advantage X have almost exactly the same spectrum) but almost 
always in combination with something else.  Has anyone else used either 
of these lights without mixing them with other bulbs and had similar 

2) I just ran through a calculation on how much K and Mg would be removed
from the water by growing plants.  The numbers were based on the
composition of freshwater algaes (Nitella and Hydrodictyon), which may
overestimate the demands of higher freshwater plants, but they indicate
that my 5 mg/l Mg and 10 mg/l K targets may have been marginal for the
growth rates I had in the tank.  Could the disappearance of the symptoms
be caused by a slight reduction in the growth rate? 

3) I gave the slow substrate circulation only about two weeks to work, 
and went on with the lighting change because I had symptoms in plants 
(java fern and bolbitis) that aren't rooted.  Just the same, could the 
sudden improvement have been a delayed effect from the circulation?

4) If the symptoms reflected damage from the lights, then WHY?  I've
fished around for a lot of ideas.  The spectra that Philips faxed me for
their ultralume and advantage X lights showed a small but significant peak
in the low UVA range, but the charts didn't show the UVB range - but it
seems that a lot of UVB would be needed to cause direct damage.  The
advantage X bulb was about 2 years old but still about as bright as a new
ultralume.  Could old bulbs produce dangerous effects? 

5) Damage from high oxygen levels in the plant tissue might mimic direct
damage from the lights, but what could cause that?  Plants normally
generate enough antioxidants to protect themselves from the toxic effects
of the oxygen they produce.  After hunting around the net for a while I
found someone who researched low antioxidant production (but not in
aquatic plants) and she told me that plants will always produce enough
antioxicants to meet their normal needs unless they receive too much UVB
(see above) or are stressed.  Has anyone else had similar problems that
accompanied very high oxygen levels or high rates of photosynthesis? 

Just to be completely clear, I don't mean to be taking a swing at 
ultralume or advantage X bulbs.  I  really like the Philips bulbs - they're 
bright, maintain their intensity better than any other bulb I've used and 
they produce nice, full colors.

Roger Miller