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Nitrogen is an element.

Phosphorus is an element.

Plants need both these and about a dozen more elements.
They use nitrogen (N) and phosphorus (P) in a ratio of about 10 atoms of N to 1 atom of P.

These elements combine with other elements to form molecules.
Nitrogen can combine with hydrogen (H) and oxygen (O) and is most commonly found in these forms in aquariums:
NH3 and NH4 are ammonia and ammonium (toxic in small amounts)
NO2 is nitrite (toxic in small amounts)
NO3 is nitrate. Reasonably safe for fish.
Plants can use all these forms of nitrogen. But, of these forms, we would like to see some NO3, and do not want to see NH3, NH4 or NO2 because these are toxic.

Phosphorus combines with oxygen and forms phosphate (PO4) which is not toxic in the levels found in the aquarium. There are tests for phosphorus, but they are not generally included in any aquarium test kit.

These elements enter the aquarium system in several ways.
Fish food has a good supply of N, P and most trace minerals (minerals that plants use in very small amounts)
Whether the fish eat the food or if it falls to the bottom and decomposer organisms eat it does not matter. The nitrogen gets turned into the forms listed above, and the phosphorus and trace minerals also get used or excreted by fish and the other organisms in the aquarium, ultimately becoming available to the plants.
Some forms of pH adjusting products contain phosphates.

When you start adding fertilizers to an aquarium it is best to figure out what you are already adding in the form of fish food or anything else.
If all you are adding is fish food you do not need a phosphorus test. Just test the NO3. Since fish food has both, then if your test shows NO3, then you can assume there is also PO4 present. You might want to adjust the fertilizer dosing so you are not adding too much of either N or P.
If the plants are using up all the NO3, then they are also using up all the PO4. So you would dose more fertilizers that contain N and P.
If you are adding other things to the water such as pH adjusting products, or any other source of minerals, then you need to figure out what they are adding to the water, and in what quantity. Then figure out if you need to adjust your fertilizer program.
 

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Discussion Starter · #4 ·
Thanks for this! So plants use P at 1/10th the speed of N?

If I never have any nitrate readings chances are my P is low as well right? This is why I am trying to figure out if its worth getting a P test, but as I said I have an extreme lack of N so the P must be at a very low level, if not stripped from the water column completely.

I use an air pump 24/7 so hopefully the P is pairing with all that oxygen to make extra fertilizer.
 

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Thanks for this! So plants use P at 1/10th the speed of N?

If I never have any nitrate readings chances are my P is low as well right? This is why I am trying to figure out if its worth getting a P test, but as I said I have an extreme lack of N so the P must be at a very low level, if not stripped from the water column completely.

I use an air pump 24/7 so hopefully the P is pairing with all that oxygen to make extra fertilizer.
Nope, the P in any form in the substrate or water does not combine with oxygen in the air.
 

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Discussion Starter · #6 ·
Nope, the P in any form in the substrate or water does not combine with oxygen in the air.
Phosphorus combines with oxygen and forms phosphate (PO4) which is not toxic in the levels found in the aquarium. There are tests for phosphorus, but they are not generally included in any aquarium test kit.
Am I missing something here?
 

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Am I missing something here?
Both statements are true.

Elemental phosphorus combines with oxygen to form phosphates, which are not toxic in the levels that are found in aquariums.

The form of phosphorus that is in the substrate or water column is typically as a phosphate anion; they will not combine with air/will not be further oxidized by oxygen.
 

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Ohh I see, if I have phosphates that means the oxygen has already converted phosphorus into phosphate right?
Sort of. Read on...

Most aquarium tests are for phosphate. This is the form plants use.

Phosphorus enters the tank in the fish food (in any of several forms, some are giant molecules) then the organisms in the aquarium (fish, shrimp, snails, and many types of microorganisms) digest the food, and use or excrete the phosphorus combined with other elements.

There will never be free phosphorus in an aquarium, in an animal body, in a plant, or in a microorganism.
It is ALWAYS combined with something. (Google for images of the reaction of phosphorus with water. Or scroll down to the last picture here:
https://en.wikipedia.org/wiki/Phosphorus )

By the time all the organisms are done with it, phosphorus generally ends up in the tank in the form of PO4, though several other forms are entirely possible. It may pass through several animals and microorganisms before it becomes available to the plants.

No matter what path it follows, the phosphorus in the fish food ends up in the tank as plant fertilizer, or else locked up in a way the plants cannot get it.
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1a)
IF the NO3 source for your tank is ONLY fish food.
No nitrogen in any form in the water, no other nitrogen additives.
THEN you can assume that there is also some phosphorus entering the tank.
Q: Please list everything you are adding to the tank. Does your tap water have chloramine?

2)
IF You are not adding phosphorus in any other form,
THEN the only source of phosphorus is fish food.
Phosphorus in the aquarium from the fish food is in about a 10:1 ratio N:p.

3)
If point 1 is true (the only source of nitrogen is fish food)
AND
If point 2 is true (the only source of phosphorus is fish food)
AND You say your NO3 is zero.
Then, I say your plants are using up all the nitrogen and phosphorus that is entering the tank in the form of fish food.

4)
If point 1 is false (there is some other source of nitrogen entering the tank)
OR
If point 2 is false (there is some other source of phosphorus entering the tank)
Then it is impossible to say how much (or how little) phosphorus is in the tank.

There is not usually much, if any, phosphorus in tap water. You can figure this out by looking up a water quality report, if you are on a public water source. Some are better than others about what they report.
As for other sources, you are the one with the labels in hand, you are the one adding them to the tank. You tell us.
What are you adding to the tank? What do the labels say? If you cannot figure it out, then tell us the product, brand name, how much you are adding and how often.
 

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Discussion Starter · #10 ·
Thanks for the write up.

1.) I do not dose nitrogen, and there is none in the water source, I target feed so 98% of food is consumed but every once and awhile a pellet makes it to the bottom of the tank (NLS pellets).

2.) I do not add

3.) I thought I was getting more nitrogen from the poop than anything, all food is consumed. Poop has nitrates right?

Here is my breakdown:
-Tap water with Prime
-Excel
-Flourish Comp.
-Iodide (for shrimps)
-Osmocote Plus (root capsules that are probably gone by now)

Thanks for another thoural in-depth reply :)
 

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Discussion Starter · #11 ·
I did buy a PO4 test, reads 0 in all my tanks (although I have low Nitrate levels). If the N:p ratio is 10:1 can we assume that 15-20ppm N should start to show a little P?

I am now using Seachem Flourish Comp, Excel, Potassium, Nitrogen, Prime, Iron, and Kent Iodide.
 
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