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re: phosphate and algae

Doug Karpa-Wilson wrote:

> I know it's the received wisdom that phosphate will limit algal growth, but
> I was wondering if anyone knew of any hard data to back that up.  I have
> read the initial studies on the krib, but they are fairly flimsy evidence.
> Have any of you played games with phosphate to test it?  Just curious.

I've "played" a bit with phosphate limitation, largely by accident.

In my experience, vallisneria were stunted by phosphate deficiency and algae 
growth was not obviously effected.

I've described that experience at length at least twice before, so I'm not 
going into it in any detail.  Briefly, I had a 10-gallon tank with valls that 
became light colored and stunted.  They formed a dense carpet with a maximum 
height of about 3 inches.  There were several types of algae growing in the 
tank at the same time.

I experimented with different types of fertilizers and found that the light 
color was corrected with added iron.  The stunting was not corrected until I 
added phosphate.  I don't recall that adding phosphate elicited any response 
from the algae

Conclusion:  the plants were phosphate limited.  The algae probably were not.

Bluntly, you can probably never use phosphate to limit algae growth in a 
healthy planted tank.  That would require that you have a source of soluble 
phosphate in your substrate and that you somehow keep the phosphate entirely 
in the substrate.  Then you would have to somehow arrange to drop the 
phosphate content of the water column to 0 (*really* 0, not just 0 on a hobby 
test kit).  You would also have to keep the content of soluble organic 
phosphate at 0.  Otherwise, the plants will suffer from phosphate deficiency 
long before the algae growth is limited by the lack of phosphate.

Roger Miller