Im running a Geo AR618. It does a pretty good job at keeping levels low, but left unmonitored, can bottom out NO3. Depending on my PO4 levels at that time, I'd dose NO3 or cut the lighting schedule to keep both from bottoming out.
I try to shoot for a 5-10 range for NO3 and 0.06-0.12 PO4.
WC...