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CO2 deficiency symptoms that mimic other nutrient deficiencies is an interesting concept. El natural aquariums don't have CO2 injection, and the plants grow fine. Other factors at work?
That is a very good question.

Plants can grow "fine" and yet a few leaves on some plants may show a nutrient deficiency. For example, some of the leaves of a single species can show a deficiency, and yet every other plant (all different species) in the tank can look perfect. Also, it is not uncommon for folks that grow sword plants (Echindorous) to trim the outer leaves when they start to look bad. Is the plant growing "fine?" Probably yes. Could it look better, also yes. Can it be grown so that the outer leaves hardly ever need to be trimmed - again yes.

In general, "El Natural" aquariums cannot support the same plant diversity as a tank with CO2 injection and with regular additions of other nutrients. Same is true in terrestrial gardening. So, it all depends on what you mean by "fine." ;)
 

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This diagram is great and i like that you added CO2 as a consideration. Ruling it in or out as a diagnostic factor may not be as simple as you indicate.

Determining adequate CO2 levels thoughout the tank can be tricky and there can always be variation in concentration even with our best attempts with flow. When a "checker" (or pH +KH measurements ) senses CO2>30ppm, levels can be lower in the vicintiy of some plants, say where there is less circulation or farther from the point of CO2 injection. Also, there might not be CO2 deficiency at all, even if checker senses level < 30 (plants can get carbon from sediment or from HCO3... and not all plants can use bicarbs).

Other diagnostic checks:

what you know or believe are nutrient levels. For example, if you have reliably tested the water column and find that non-C nutrients are available(and in good proportions), then you can look to CO2/flow or light as a potential cause.

Not my area of expertise, there are also the issues of nutrient excess (toxicity), which may provide visual Q's or simply bad ratios (where one nutrient blocks the uptake of another).


Unfortunately, i dont have simple suggestions for you to implement.
 

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Column levels of nutrient toxicity are very unlikely; Hoagland stock solutions used in many scientific experiments are 10x EI dosing in the same sort of parameters.
Toxicity can also come from the sediment. This situation may be of interest to followers of "the natural aquarium," aka "El Natural" here on APC. Soil based sediments can create a wide variety of plant toxicities, particularly if lots of water changes are not done (as suggested) during the initial setup. Some of these toxicities can create a nutrient imbalace for the plant which are not toxic to the fishes. I recall Diana talking about zinc toxicity and it may even be covered in her book.
 
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