nutrients.... either micro nutes (Seachems Flourish) or macro nutes (which TPN+ containd both micro and macro) but cannot get it ANYWHERE in the US

......... You should be only dosing C02 (Excel) if you have high enough light, but then you would need to be dosing nutes as well... If you do not see any nutrient deficiencies while dosing liquid C02 than most likely you do not need to be dosing it... Like I said earlier, I have pretty low light ( 30 W for 55 gallons ) and I am only dosing micro nutes (Seachems Flourish) twice a week and am getting substantial growth. I do not need to dose macro nutes because my tank is stocked enough to the point where the fish provide them...
More C02 means faster growth which also means a higher demand for nutrients BUT if there is not enough light, higher amounts of C02 mean nothing
EDIT:: You should look in the planted section through a couple of topics and you should see what I'm trying to say....
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GOOD LUCK