A good question: Why is Tropica nutrition so awesome? Why do copy cats without Urea not perform as well? Why does Urea overdosing not work if Urea provides CO2 and Nitrogen via enzyme Urease -- I mean CO2 without worrying about gases!?!
Stuff we need:
1: Nitrogen drives growth (you can increase your own metabolism by increase your protein - nitrogen - intake ... and I reckon not ONLY due the thermic effect).
2: Fix a species. For each "amount" of N, there will be a unique amount sugar - neccesarily CO2 - that is required to metabolize it. It has to use it, due to gavage water column feeding of NO3 (see the first post photo).
3: An example of a species that needs more CO2 for each N is ... hopefully no surprise to anyone: Rotala Macrandra.
This is why people say these "sensitive" species do better under low NO3 in the column AND require more CO2 (I mean we can't control NO3 liberated into the column, come on).
More CO2, localized at the leaf (via good flow), WILL solve every issue. But to gavage feed NO3 and then dope the tank with high levels of CO2 is cruel to the livestock. Not all livestock, ofcourse, but the ones that are not used to such a low pH and so much free CO2.
Ok, now that we see this balance of N and CO2, why is a common advice to get proper plant forms on MORE sensitive species to reduce N? Well, because to get the CO2 you need to match your Nitrogen forced feed on those particular species will cause immense amounts of discomfort to livestock. Look at the photos of "beautiful tanks", you can tell how people dose by looking at the fish ... or the absence of fish entirely.
Reducing light will work ... but we established above how important it is NOT to restrict light.
If you restrict N (and P) too far, the tank isn't stable ... overfeeding of fish will cause unhealthy plants, algae etc (and a good exercise would be to understand that statement from what I wrote in the post). Sound familiar? Old school ideology.
So, dosing some N and P is good. Too much NO3 and we go back to the above, driving CO2 demand.
Ok Why is Tropica so great? They dose Urea too ... CO2 pack-a-punch. So, classic, get rid of all NO3 and dose only Urea ... genius. BUT it won't work. Why? Same thing.
If we think about evolution - the plant would have ALWAYS been in an environment where Nitrate and ammonia are present. And the transportation of these nutrients during low tide, high tide, etc etc etc is different since you need to use active transport over the root etc.
So, the "smartest" plant would assign jobs to both Urea and NO3. Welcome Tropica: No3 and Urea over the week. Genius. This is why Tropica tanks look so beautiful. It is also why ADA looks so beautiful --
How can we circumvent this with only NO3? Stock heavy and feed lots. I hope this sounds familiar to many readers.
To understand the latter, we need to connect that Nitrate requires CO2 to convert it to Ammonia ... so if you give no ammonia, you need EVEN MORE CO2 to compensate (and if you have no nitrate and just ammonia); of course the plant can do it ... or it wouldn't have survived.
We could even analze the lines of fertilizers ... the ones for "form and colour" are always leaner in N (and P) ... and if you look at cases where people switch from richer lines of the exact same fertilizer to leaner varietals (so they have issues with a richer of the same brand), it is always to alleviate stunting, formation, compactness, etc etc ... all common CO2-related/nutrient-related issues (and I put both because they are all the exact same thing).
The best advice (unless you want a wild ride): Buy a pre-made fertilzer (or mimic it properly) and dose it as per the instructions.
Oh, one more thing, in photos if we look carefully at the fish species and contrast it to the plant form and plant selection and observe the "state" of them ... they are always well-matched for pH/CO2 OR one of the living things are suffering.
Also ... every method can work and provide a healthy tank.
Cheers,
Josh