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[APD] Re: Begging limitless questions

On Tuesday 16 September 2003 21:11, Scott H. wrote:

> Yep, same goes for lighting too, and the rest, right?  So
> why must there be a deficiency?

Some years ago in this post


Dave Heubert offered the tidbit that under ideal conditions fast growing 
aquatic plants are capable of doubling their mass every 4-6 days.  Personally 
I may have seen plants grow that fast briefly, but I've never seen any plant 
sustain that kind of growth.  Most of our aquarium plants grow much more 
slowly than that and it's a good thing.  Personally I wouldn't want to remove 
1/2 ot 2/3 of the plant mass in my aquarium every week just to maintain the 
status quo.

Perhaps in some cases the growth rate is limited by the genetic makeup of the 
plant.  I doubt that accounts for very many limitations because if it did 
then we would never see variations in the growth rate.  The growth rate would 
be a predetermined constant set by the plant's genetic code.

I theorize that almost any healthy-looking plant we see in our aquariums has 
its growth limited by light.  In some cases perhaps the CO2 supply is the  
limiting factor.  I maintain that theory plainly and simply because every 
nutrient shortage has a disease-like deficiency symptom.  I don't think a 
gardener can limit plant growth with nutrients without eventually seeing 
those symptoms.

I think this also goes to why George B. protests so about the recent trend of 
recommending high light in aquariums.  High light levels do little more than 
lift the limit on growth rates, force greater demands on CO2 and nutrient 
levels and increase the likelihood that nutrient-related problems will set 
in.  Why go out of your way to invite problems when there is so little to be 

The question about how a light-limited plant looks may have a simple answer; 
it looks healthy and well-nourished. There are obviously lows at which this 
is no longer true -- where the light is too low for plants to grow.  That 
point varies from species to species.  A healthy-looking aquarium always 
contains an assemblage of plants that can grow well within whatever light 
limits the aquarium imposes.

Much of this might also be said about plants with growth limited by CO2.  
Light and CO2 are so closely linked that their effects can be difficult to 

Roger Miller
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