I would say that it depends heavily on the corals themselves. Some are perfectly fine with a "dirty" tank while others won't stand for it at all. In this regard it is a bit of trial and error. I've run my tank with 5ppm nitrates as well as near zero as well as above 10. I've had SPS survive all 3 levels of nitrates but I've also had SPS die at all 3 levels. Over last summer my skimmer died and my bioPellets ran out so I was running a fairly dirty tank. I had a lot of SPS that were fine, but I also had a few completely brown out and some even died. When I turned the tank around (new skimmer, new pellets), all my browned out corals regained their colour. All my dead corps remained dead. I'm probably running somewhere near 1-3ppm nitrates and near zero (but not exactly zero) phosphates at the moment. I've found my best results to be 1-5 ppm nitrates.
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