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Phosphate levels

Karen wrote:

If you supplement with nitrate, you can easily induce a phosphate limited
state assuming other macro and micronutrients are in adequate supply.  I
don't think I've ever seen an algae problem caused by phosphate limitation,
and you ahve to starve the tank of PO4 for quite a while before you see
much more response than retarded growth.

Again, I think that limiting K would cause problematic "symptoms", but I
see no reason to limit K, as it is not implicated in algae problems even at
high levels, there is a wide range of acceptable concentration, and it is
easy to dose right along with the nitrate.

>My point is that as long as there's a range of nutrient concentrations
>that we can work in we *should* be able to have nutrient-limited growth
>without having diseased-looking plants.  

As long as we pick the right nutrient to limit.  I still favor P.  I'm not
sold on Fe, though I can see how it might work if you are _only_ limiting
it in the water column with a plentiful supply in the substrate. (as in the
soil based tanks that Diana and Steve P favor)

Here are some observations on my particular conditions (which are far from
I have been trying to acheive a P limited tank for some time but have had
to battle with PO4 levels of 5ppm in my tap water.
My tap KH is 2 and initially 18 months ago I had a low light tank with
anubias , crypt affinis and java fern , I added clay balls into the old
gravel and used an iron supplement infrequently. When the water company
started to add PO4 I had a massive outbreak of BBA then following some
earlier threads I increased lighting to around 3 watts/gall and BBA died
off completely but was replaced by BGA. I started adding fast growing
species which seemed to grow only slowly but looked healthy but the BGA
persisted. Massive water changes only made the algae grow faster. Adding
DIY CO2 brought pH to around 6.5 so I started to add small amounts of
Calcium carbonate as powdered limestone then changed to powdered dolomite
to bring KH to 6 , I also added a few jobes sticks around my C.Affinis
which did not seem to like the changes I had been making . Still plenty of
BGA but most plants were growing faster , but no pearling. Trying additions
of Tetra Florapride and KNO3 and using as much rainwater as possible with
only small amounts of tapwater brought about a much reduced BBA level  and
good growth but my nitrate levels soared to 100ppm so I started to dose
with Potassium Carbonate , Nitrates are around 20ppm and I have changed to
TMG and PO4 is around 2ppm. I have not had any thread algae( some
introduced on E.tenellus disappeared) or BBA and have an odd small patch of
BGA or slimey green appear on the back glass or  Echinodorus leaf
occasionally , but not enough to detract from the overall appearance.
Soooo... in view of the comments above and assuming my tank is balanced ,
its been like this for around 3 months and I have to prune every couple of
weeks, and as I add only 10-15 ml TMG per week  does this sound like Fe
limited? (I keep trying to get a reasonable priced Iron test kit  to give a
better indication,) and if so if I increase TMG levels will this result in
an algae bloom?At the moment I am happy with the tank as it is but will no
doubt soon want more difficult species and be tempted to change the regime!
Thanks for your input,

John Holt
jdholt at compuserve_com