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growth limits

Some thoughts for those *not* too busy getting ready for the Amazon

Most of us have a goal to keep a lot of robust, attractive plants.  To
meet that goal we put enough light on our tanks to illuminate small
offices.  We add CO2 to increase the plants' size and growth rates.  Then
we supplement nitrogen, potassium, minor elements and trace elements
because if we don't the plants become yellow, corroded, weak and
sick-looking.  Then in the nutrient soup we create we have to find a
delicate balance of conditions or we are overwhelmed by algae growth.

But what if we back off the light levels, so that light and not the
nutrients are growth limiting?  Then we have to worry relatively little
about nutrient shortages and their symptoms.  Growth is slower and
necessary maintenance is reduced.  Maybe there are some plants we can't
grow, but I've seen even reputedly light-hungry plants growing well in
moderately lit tanks.

What is the level where light limits most plant growth rates, but we can
grow the most diverse group of plants?  I have tanks with a little more
than 1.5 watts/gallon (60 watts, 38 gallon tank), 2 watts/gallon (40
watts, 20 gallon tank) 2.5 watts/gallon (50 watts, 20 gallon "long" tank)
and about 3 watts/gallon (160 watts, 55 gallon tank, two 10 gallon tanks
w/30 watts).

I have no end to aesthetic problems in the 55, but the two 10-gallon tanks
are probably carbon limited and they don't consistently show any
unattractive deficiency symptoms.  The tanks at 2 and 2.5 watts/ gallon
maintain lush growth with minimal attention.  The tank with 1.5
watts/gallon is probably also carbon limited and it grows very few plants.

It appears to me that reduced light is the way to go when faced with
multiple and ongoing deficiency symptoms.  But maybe there are other
reasonable alternatives?

Roger Miller