Signs of genuine macro nutrient deficiency


Guru Class Expert
Oct 26, 2007
Brisbane, Australia

I'm not actually having any nutrient problems at all - just thought I would post this for the sake of discussion, to better my understanding.

I've now learnt that it's usually a CO2 limitation that causes a whole host of problems, not an excess nutrient/not enough light issue as many books etc will have you otherwise believe.

But for a moment lets flip this on it's head....

Imagine we have an aquarium that is ticking along very nicely. Photoperiod is good, CO2 is non-limiting, nutrients are non-limiting (EI).

Now, let's NOT dose any nitrogen.

Given that Light, CO2 and other nutrients are good, what are the true signs of a nitrogen deficiency?

Similarly, if we only hold off dosing Phosphorous what we be the expected signs of deficiency?

And again for Potassium?

Don't want to discuss micros (...yet). ;-)

And, to keep things simple, lets assume there aren't any fish in the tank, so no fish food etc. Nutrients only come from EI dosing.



Plant Guru Team
Lifetime Member
Sep 23, 2007
South Florida
Hi Scott,

I always think of it as a chain reaction as each of these elements work together to form other links in the chain.....

I cannot answer your question but think you would need a rigorous control to ensure that the symptoms are 100% caused by the limitation of X nutrient.

So if N is limited and it affects the K or P is it now more the reaction of the K or P that is more the cause, or the original reduction of N?

I hope/am sure Tom will chime in for this one :)

Tom Barr

Staff member
Jan 23, 2005
To address CO2 issues and other factors, many use hydroponics to address this test.

You can find plenty of references for many plants done this way/method.
Many deficiencies look similar and are different on different plants.
Adding to that, no tables showing deficiencies I've ever seen.......include dissolved CO2;)

It's(dissolved CO2) just not studied in an agricultural sense.
But most aquarists simply glaringly overlook it.

This is extremely short sighted.

Tom Barr


Lifetime Charter Member
Lifetime Member
Mar 12, 2009
As Tom said, there aren't any that include CO2. I've looked a long while for it.

Here's where I refer to every now and then when I come across strange symptoms:
Nutrient Deficiency

I had another one that was agricultural, and while the pictures didn't always match, the description of what happened to the plants was excellent, even for trace elements. I wish I could find it again; if I do, I'll send you the link.