I'm really going to need to start a journal of all this. The short answer is, the balance of Nitrogen/Phosphorus/Potassium (N/P/K), Co2, & light, controls everything. "Low Tech" generally refers to a tank with no Co2 added, so you need to add less nutrients (N/P/K) and light. The "High-Tech" tanks with Co2 injected, need more light and nutrients. And the N/P/K itself needs to be balanced. If you have high nitrates but no phosphorus or potassium, the plants can't utilize all that N without the others, no matter the light or Co2 . The separate issue is, especially if you have high powered lights, with a nutrient imbalance like that, you'll probably end up with some type of algae bloom. There are more than a few species of algae that can take advantage of each nutrient imbalance. That's where the thought that excess phosphates and light (especially natural sunlight) cause the green algae that grows on glass comes from. This is not untrue, but it doesn't hit the core of the issue. If you took the exact same tank, boosted the Co2 and made sure the other nutrients balanced out, plants in the tank would more than absorb the phosphorus. It's not so much too much Phosphorus, as it is an imbalance of all factors. If you can't get the others up, you need to balance down to the lowest factor.
FWIW, the K, that's what is really lacking in most non fertilized tanks from what I've seen. There is basically no way it'll end up in the tank without it being added specifically, and without a moderate amount, the plants are limited to the amount of P & N they can utilize.
And that is before you add in all the lesser factors. Like iron, calcium, magnesium, etc etc. They play a smaller role, but again, if you have too much phosphorus and iron, that changes the game. Then there's water flow.
The whole point is, the learning curve on this is kinda steep. I was rocking perfectly with this new planted tank for almost three months. Around 10 days ago I started growing some type of green hair algae on the leaves of a few plants in the top 4" or so, near the center of the tank. I ran one day with the lights off, reduced the time the lights were on full power, reduced the power of the lights by 10%, and knocked back the K & Fe I was dosing. Seems to have died off. And yet, I am not 100% sure what combination of all that has made the difference. I'm pretty sure it was mostly the lights, since the algae growth was mostly at the top near the lights. So again, I need to rebalance everything else, to make sure all is in tune with the new lighting. Lol...
