I am with you Kien, I like to flow different rates too. once a while I have no flow at all.
All jokes aside, I run rowaphos with my carbon off the same pump for my pellets and UV. I was always getting a 0 ppm reading from Hanna but since I got the ultra low range its showing up at 13 ppb. I will be running a bit more GFO ratio to carbon see if I can bring it down to 0. I really dont think its critical to have phosphate at 0 but since we are introducing phosphate into the tank all the time it wouldn't hurt to try.