Depends on what kind of SPS
you're wanting to keep..... Montiporas, some milleporas, and birdsnest can tolerate slightly dirtier conditions, but not by much. You're going to find yourself doing very large water changes much more frequently to maintain the higher standard of water conditions that a "full on" SPS
environment is going to require. At this point, I'd highly recommend a sump with a good skimmer & refugium with macro algae for nutrient export.... otherwise you're going to find yourself buying salt by the skid.