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General hardness

On Friday, 5 Feb, 1999 Alex Pastor axked what the lowest Gh is that will
still support healthy plant life in an aquarium.

My tap water carries about 21 ppm (just over 1 degree) of total hardness
according the the city and my test kits read 2 degrees.

I've had likely calcium deficiencies in two Echinodorus spp. (osiris and
bleheri/amazonicus) and in Val.  The main symptom is deformed new growth.  
Boron deficiency is reported to cause the same symptoms, but my tap water
carries quite a bit of boron.  I've never (before, anyway) had problems
with any of our common stem plants - even Egeria densa which is reputed to
be a "hard water" plant grows well.  I've got a couple odd looking new
leaves on a giant hygrophilla; hence the paranthetic reservation above.

I figure this puts 21 ppm total hardness (1-2 degrees) near the
minimum where all of our plants will grow without problems.  But there's a
catch or two.

1)  All of my deficiency problems appear in one tank.  I can transplant
affected plants into another tank and the symptoms will disappear.  Even
in the problem tank, plants don't consistently show symptoms.  Val in
particular has grown very well in the past and only recently (after I
added new substrate material) has it had problems.  All of my tanks get
the same 15% water change per week and I use tap water in all of them, so
the different behaviors are something of a mystery.

2)  I imagine there some RO users and some folks up in the Pacific
northwest who are growing the same plants in water with even less general
hardness.  The amount of sodium in the water may alter the minimum amount
of hardness necessary for good growth.  On a guess, low sodium
concentrations might allow plants to stay healthy at lower total hardness
levels.  I have relatively high sodium concentrations in my tap water.
The nature and age of the substrate may also play some role.

This has probably been my single most tenacious problem growing plants, so
I am

Roger Miller
eagerly anticipating further discussion.