I'm running two 14000k phoenix HQI, 7 hours, PO4 is detectable, nitrate unsure, my API kit may be off. Had it for 10 months but as I hear you need to shake it for a long time to be accurate, I think the kit is unbalanced now but it reads 40 ppm. I have about 2" of sand in DT and a 4" bed of MM in my fuge ( 11"x14").
there are different types of flatworms. The clear-white ones that you see on the glass and some softies are harmless.
there are others that will wreak havoc on your corals. I think flatworms usally leave bite marks on sps, if you take a piece out of the tank, and let is sit in the air for a couple of minutes, the flat worms are easier to spot.