Plants are melting


I have a problem with the plants in my 20H.  I have an amazon sword
that has started melting away (the longest leaves are turning
transparent, but keep there structure), the leaves of my java fern are
turning transparent, and are melting to mush, the micro sword is
looking puny, and the hygrophilia lost most of the leaves and is not
doing much right now.  This has been a fairly slow and agonizing
process - over a period of about 2 weeks so far. My tank conditions are
PH: 6.7, KH: very low (~1 degree), GH: very high (>12 degrees),
Ammonia, and Nitrites/Nitrates within normal, Temp: 80.  Tank load:  9
cardinal tetras, 2 corys,  2 clown loaches and 2 otocinclus.  I realize
the GH is really high, but I can't figure out why.  The conditioned tap
water used for water changes is very soft (low KH, low GH), and I've
only got a few rocks in this tank, many of which have been in this tank
for years.  All of the fish are doing fine, in fact the otocinclus have
bred, and I have several baby otocinclus running around the tank.  I
just started doing water changes on this tank about 2 months ago (it
had been sitting in a quasi-biologically stable state for about 2 years
with 1 clown loach and tons of java fern), and recently replaced the 1
15W bulb with a tri-lux.  I am fertilizing once a week with Kent FW
Plant Supplement.  I realize this is not an optimum plant tank, but I
have been able to grow plants before (when I cleaned it out, I removed
a 5 gallon bucket full of java fern).  Could there be something in the
water that is not measured from the tap, but gets released in the
aquarium environment, or should I suspect one of the long-time rocks of
suddenly leaching impurities into the water?  I am desperate for help,
and the fish stores around here are all stumped.  Thanks in advance for
suggestions, comments, and help!

David Cooke

* Emory University Hospital       email: david at joshua_medimg.emory.edu       *
* Division of Nuclear Medicine                                               *
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