I do not believe it to be water issues because the only thing that ever dies in my tank is my cories. Also I haven't had a death in cories for a very long time, though, so I think I'm getting better at feeding them. Despite the fact that the schools are incomplete, the fish still seem very happy. They are all very active and do not just swim with their own species. Never have I had an issue with them. Of course I understand that they'd be even happier with complete schools.
There really isn't a place where my swordtails can't get to in the tank.