Martin,

To answer your question straight away, shoot for a 3 - 5 ppm concentration
of nitrate in the water column. More and your fish might be affected.
Higher plants need nitrogen (from nitrate) to survive. When you have plants
and can't measure nitrate in the water, you are seeing a condition where
your plants have scavenged all the available nitrates. While that seems
good, it means that nitrogen is the limiting macronutrient. With no more
nitrogen, the plants stop photosynthesizing. That's not good, since
phosphorous is probably still available, and though it might be available in
small amounts (ie 0.25, ppm I assume), that's still enough to cause algae
problems.

Nitrogen (N) is a macronutrient required by aquatic plants, as are potassium
(K) and phosporous (P). Phosphorous can be a problem child, and an excess
amount of phosporous in the water column will create algae and cyanobacteria
outbreaks. What's interesting is that some algae (specifically blue green
algae (cyanobacteria)) can fix nitrogen from the atmosphere. So it can
thrive with very little or no available nitrogen in the water. Higher
plants require lots of nitrogen to survive but far less phosphorous; just
look at recommended nitrogen to phosphorous (N:P) ratios for higher plants
(it ranges from 10:1 to 16:1; all this is from the Krib website). Higher
plants are more efficient at photosynthesis, so with enough nutrients they
can outcompete the primitive algaes. Cyanobacteria, on the other hand, can
survive with very low N:P ratios (probably less than 3:1). Given the
nitrogen fixing ability of Cyano. (higher aquatic plants can't do that), you
have a recipe for outbreak if your nitrate levels read 0.

By dosing with nitrate, you provide a small surplus of nitrogen for your
plants. True, the algae will use it too, but the plants consume it much
faster (hence the higher N:P ratio). As long as they have nitrogen (and
potassium and micronutrients too), they consume phosporous, and that's the
whole idea. You want to have the higher plants strip the excess phosphorous
from the tank so the algae can't get it. If you have algae, adding nitrates
(and anything else your plants need, including light and CO2) will cause the
algae to starve.

To sum things up: aquatic plants need light, CO2, macro (N, P, K) and
micronutrients (Fe, Mg, Ca, etc) in sufficient quantities AND proportions so
that the elements that can cause problems (most notably phosphorous) are the
limiting element to photosynthesis.

I use potassium nitrate (off the shelf stump remover here in the US) for my
nitrate additive. You can buy lab grade stuff at a scientific supply
company, but keep in mind KNO3 is an oxidizing agent and could be used to
make explosives. I used to mix KNO3 with powdered aluminum to make my own
fireworks way back when I was a boy.

For potassium, I also add K2SO4 (potassium sulfate). This is "pure"
gardening potash, and the sulfates provide sulfer for the plants. This
should all be familiar from PMDD recipes found at the Krib website.

Hope this helps,

Jeff
Raleigh, NC USA



"Martin" wrote in message
...
Ok, I have a big problem with this. For years I have been trying to get
Nitrates down (in smaller tanks) could not get rid of the stuff.

Now I have a 250l tank with enough light and CO2 injection and a 100%
Fluorite substrate. I now find that my Nitrates and Phosphates are low
(well
actually 0 and 0.25 respectively).

I find from posts I have read here - that I should be dosing with Nitrates
to get the plants to grow.......this is a jedi mind trick right!!!

OK, so if I have to put the bloody stuff back in......then....

What should I use?
How much? (for a 250l tank)
Can I get it pre-packed from a store (initially - until I get the
confidence
to make it myself).....
and more to the Point WHY do I have to?......I thought that by buying
Fluorite - the plants would have all they need straight from the
soil.....I
am loathed to put stuff into the water. In my opinion that is just asking
for algae.

I have Cabomba, Heteranthera, Vallis, Sagittaria, Eleocharis, 2
Echinodorus
Harbich

Thanks in advance

Martin - I can't believe I am actually trying to put Nitrates back into an
aquarium!!!!!!!!!!!!!!!!! This is mad - right.