My NO3 was 20ppm and PO4 at 0.04 prior to putting my Atlantis on I don't consider those figures "low" I was running another much cheaper ATS but the lighting wasn't up to the job and growing algae quickly enough to bring my nutrients down to acceptable levels, well acceptable to me and where I wanted them to be.
The idea behind an ATS is to outcompete GHA outside of it as in the DT using lighting targeted at growing algae IE lots of red in the spectrum. BTW I don't run UV.
Water is passed over the screen in a thin layer rather than through it. The interface of water and light creates ideal conditions to grow algae much better than in the DT and in doing so reduces the nutrients. The whole idea is to grow algae where you want it and where it can easily be harvested.
I grow at least 3 types of algae on the screen of my ATS not just GHA ones of those algae I have not been able to identify the other is Ulva intestinalis a very quick growing algae which appears to take up nutrients extremely quickly EG it grows on my screen at the rate of at around 1.5" in 24hours. Outcompeting for the available nutrients is the key to keeping GHA out of the DT or reducing/eliminating it in the DT.
