I don't think SPS have a polluting effect, I just think that they thrive under very clean, low nutrient conditions, whereas you LSP are filter feeders and prefer a bit more "dirtier" water. I find it difficult to keep LSP now that my tank is mostly SPS, they tend to shrivel up and expose their skeletons quickly. You can probably find a midpoint to where they're both happy, but I haven't been so lucky. Rics, shrooms, and zoas really look ugly in my tank, my acans, toadstool, and chalices are unaffected so far.