I have been doing some extensive experimenting with this. Granted I am not a scientist and my testing supplies are hardly super accurate. With all this stated my findings are sound for me and enough for me to express what i have seen.
Currently I am running 2 tanks with less then 5ppm of N in the water(probly closer to 2ppm...judging by my calculated dosing). My P is less then .5pmm also.
I started this experimenting with the same question as unirda. Another question i always had about amano's stated levels was his co2. Using KH/PH his co2 was always so so low. So i set out to see what i could accomplish with these levels.
Tank specs:
75 gallon with 220watt PC lighting for 10 hours.
KH 3
GH 10ppm
the tank uses flourite so i was able to use a controller without KH interference.
I ran my co2 at about 8-10ppms
dosing left me at a consistent >5ppm of N and >.5ppm of P at the end of each week. I ran this setup for 6 weeks. All plants grew full and healthy.
I also found that i had crystal clear water.....and
ZERO GSA. VERY SLOW GROWTH, but very healthy.
50 gallon with lower light 110 PC lighting for 10 hours
kh 3
gh 10ppm
aquasoil for substrate. I estimate my co2 to be around 10-12ppm
dosing is even less in this tank. I dose about .5ppm of P a week and 5ppm of N a week.
This tank is growing even better. N in the water column is almost unreadable.
So it is absolutely possible to grow beautiful plants in very limited conditions. IME, i prefer the slow growth. I am a scaper, not a farmer. The slower the better IMO.
Over the next 6 months i will continue to tweak the system to see the effects. Recently, I planted the 75 with root tabs in three sections. Interestingly, a huge amount of NPK leeched into the water column. The fish seemed fine, but the 30-50+ppm of N in the water stunted almost every single plant and killed about 25% (some rare stuff unfortunately....sorry max) in my finely growing tank. I know I know.....more co2. Honestly, i am not playing that game.
Inspired by kekon, amano, cp1007, and edwards co2 myth thread, I am attempting to grow plants slowly and healthy while providing a low co2 environment for my critters.
The only draw backs have been some very slight algae issue......until i added amano shrimp. That ended that very quickly. And also very slow growth.
Sorry that my post was less the scientific.......but this is what i am finding and i am more then confident in my eyes
Let me know if you have any comments.
jB