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PMDD regimen

A reader from Australia writes: 

>   I increased the lighting on my tank to 4 x 40w tri-phosphor daylight 
>   tubes.  I also got the latest Sears & Collins [sic] PMDD formula and 
>   mixed up a brew.  I'm now running yeast C02, reasonable light and daily 
>   dosing drops. Growth is good.
>   But... If I'm not sadly mistaken, about three months out I'm seeing 
>   the growth rates level out a bit.  I'm also not seeing the visible 
>   bubble streams of photosynthesis so much although PH drop and air 
>   bubble movements tell me CO2 is happening.  
>   I was wondering about nutrients.  Is PMDD only part of the equation.  
>   Should I be replenishing substrate nutrients here?  Given your web 
>   page referred primarily to the "dupla" scenario (too expensive here in 
>   Oz) I wasn't quite sure about how to apply the data to my el-cheapo 
>   scenario.

I think it may be a matter of balance.  Before your changes, things had adapted 
to whatever light and nutrient levels where available and growth rates matched 
the limiting factor (light, CO2, iron, whatever).  Most likely, other things in 
relative abundance were stored in the plant's reserves or were seqestered on 
binding sites in the tank (detritus has some CEC and can capture trace element 
ions just like laterite, kitty litter and other substrate additives).

When you changed things, a different limiting factor came into play.  You saw a 
change in growth while the plants adapted to new growth levels, perhaps pulling 
some things from reserves.  Perhaps now that the old limiting factor was in 
good supply, the reserves of whatever allowed greater growth that the long term 
supply of it would support. Now you've reached a new equilibrium with a 
different limiting factor, perhaps due to the reserves of that element being 

"Your Mission, Mr. Phelps, should you choose to accept it, is to find that 
limiting element.  This e-mail will self-destruct in fifteen seconds". 

Note that the PMDD regimen is not a magic formula but a process.  The published 
PMDD formula is the end result of that process for the specific aquarium 
situation of Sears and Conlin (fish, plants, substrate, tap water, lighting and 
many other factors).  To make it successful in YOUR situation, you have to go 
through their process of adjusting various bits of the formula and observing the 
long term effects. Their goal was to make phosphate the limiting element, thus 
preventing algae (their conclusion).  The interesting and counter-intuitive side 
effect was that they had to ADD nitrate, which was heretofore considered "bad". 

Reread their first posting in the archives that describes the process.