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"Hardness" uptake in an aquarium and a related question

I was curious about harndess changes in a planted tank over time,
especially after several recent postings. 

I tracked alkalinity, calcium and magnesium hardness in our three tanks for
a week to see what happended. I was somewhat surprised. I expected
alkalinity to be reduce some due to nitrification and Ca and Mg to be
reduced due to plant usage. I also half expected Ca and Mg to be used in a
4:1 ratio. 

Also, since water evaporates pretty quickly in our climate, we top off the
tanks daily with tap water and that addition needs to be considered. Our
water is soft but does have some hardness. I measured our tap water with
LaMotte test kits (all three expressed as CaCO3):

  48 mg/l alkalinity
  28 mg/l calcium hardness
  10 mg/l magnesium hardness 


"120g tank"
-----------
Note: We just added two new 175w MH bulbs 4 days before we started tracking
hardness - fastest possible growth?

370 liters total water capacity
26.5 liters of water added over 7 days (an open top tank)

Alkalinity
start: 104 mg/l (38480 mg total)     end:   112 mg/l (41440 mg total)
added:          ( 1270 mg total)     delta: gained 4.6 mg/l (1690 mg) 

Calcium
start: 100 mg/l (37000 mg total)     end:    96 mg/l (35520 mg total)
added:          (  742 mg total)     delta:  lost 6.0 mg/l (2222 mg) 

Magnesium
start:  24 mg/l ( 8800 mg total)     end:    20 mg/l ( 7400 mg total)
added:          (  265 mg total)     delta:  lost 4.7 mg/l (1745 mg) 


"100g discus tank"
------------------

300 liters total water capacity
13 liters of water added over 7 days

Alkalinity
start: 76 mg/l (22800 mg total)     end:   84 mg/l (25200 mg total)
added:         (  624 mg total)     delta: gained 5.9 mg/l (1776 mg) 

Calcium
start: 42 mg/l (12600 mg total)     end:   42 mg/l (12600 mg total)
added:         (  364 mg total)     delta: lost 1.2 mg/l (364 mg) 

Magnesium
start: 18 mg/l ( 5400 mg total)     end:   18 mg/l ( 5400 mg total)
added:         (  130 mg total)     delta: lost 0.4 mg/l (130 mg) 


"100g dawrf neon rainbow tank"
------------------------------

300 liters total water capacity
13 liters of water added over 7 days

Alkalinity
start: 76 mg/l (22800 mg total)     end:   76 mg/l (22800 mg total)
added:         (  624 mg total)     delta: gained 2.1 mg/l (624 mg) 

Calcium
start: 64 mg/l (19200 mg total)     end:   64 mg/l (19200 mg total)
added:         (  364 mg total)     delta: lost 1.2 mg/l (364 mg) 

Magnesium
start: 16 mg/l ( 4800 mg total)     end:   16 mg/l ( 4800 mg total)
added:         (  130 mg total)     delta: lost 0.4 mg/l (130 mg) 


Summary:

All the tanks gained some alkalinity gain over the time period. I was
surprised by this. I'm not sure of possible sources; perhaps fish food. The
dwarf neon tank gets less food than the other two and the measured
difference in that tank can be accounted for by the gain from top off
water. The discus tank has the highest nitrates of all three (about 15 mg/l
versus 5-10 mg/l).

All tanks showed calcium and magnesium loss. However, the "mg/l" loss is in
the range of the test kit accuracy. The 120g with new lights showed a
larger loss than the other two. 

However, compared to calcium losses in a tank using peat (mentioned by
Steve Pushak in an e-mail), the tanks seem very stable, especially when
there is a gradual addition of hardness via top off with tap water. 

Follow up question: 

It seems that plants use very little calcium and magnesium compared to
recommended concentrations in the water. I believe this also holds true to
iron and perhaps CO2. Do plants require a higher concentration of nutrients
in the water and/or substrate to drive uptake processes? Is this much like
respiration, where there has to be a differential in concentration before
exchange takes place? 

The general "rule of thumb" for iron concentration (0.1 to 0.15 mg/l) is
much higher, I believe, than actual usage. Is there a threshold for various
nutirents below which the plants can't utilize them?   

George Booth, Ft. Collins, Colorado (booth at frii_com)
  Back on-line! New URL! Slightly new look! Same good data!
    http://www.frii.com/~booth/AquaticConcepts/