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Re: Algae, algae eaters, algae

Tom Wood wrote:
>  I guess I have to question the wisdom of adding more critters to a tank to
>  combat an algae (read: nutrient) problem. The implied strategy of the
>  Sears-Conlin PMDD paper is to increase everything EXCEPT phosphorous (and
>  maybe nitrogen) in order to drive the tank into a P (and maybe N) limited
>  condition. I seems to me that adding more fish/shrimp/snails can only
>  increase P and N since they must eat, and therefore must add to the P and N
>  equation. 

    I think this "wisdom"is counter-intuitive to a lot of people.  It is 
important to remember that it is not the number of animals in the tank but 
the amount of nutrient (I'll use nitrogen here) that is added to and removed 
from the tank that controls nitrogen levels.  For instance, if a snailless 
tank that gets a given amount of food daily (N input) and a given amount of 
pruning weekly (N export) has snails added (in this example the snails 
neither grow nor die) but the feedings remain the same, the available N will 
not change.  
    Sure, the snails are eating and excreting, but it is what they are eating 
that is important.  This is because the snails are eating: A) fish food that 
the fish missed and dead plant and algae material.  Before the snails,this 
stuff would have been converted to plant-available N compounds quickly anyway 
by bacteria.   B)  Fish waste- the snails reprocess it more efficiently than 
the fish do.  Still no net change in waste N compounds.  C) Algae. This is 
where my "unchanged available N" statement is off, but it's a worthwhile 
trade.  The N compounds in the algae are converted by the snails into free 
nutrients for all plants.  In other words, there _is_ now more nitrogen 
available, but it came entirely at the expense of living algae.  
    To use Tom Barr's team analogy, it's like grinding up some of the algae 
team and using it to feed both the plant and algae teams.
    To use another example, imagine a fishless plant tank with aphids.  The 
aquarist throws in fish to eat the aphids and figures he no longer has to add 
N anymore, because the fish are excreting it, right?  Well, maybe.  The N he 
used to add went into plant growth and came back out of the tank as plant 
trimmings or as aphids flying away.  If the aphid infestation is severe  
enough to stop net plant growth and the fish allow no aphids to escape the 
tank , then it works.  If, however, plant growth still requires pruning , and 
some aphids fly away,  the tank suffers a net loss of N.  No matter how well 
fed those fish are on aphids, plant growth will wind down to a stop.

    HTH
    Pierre Gagne
    Kensington, MD