OK, I'll preface this by saying that I don't keep spotteds, and have never kept spotteds. BUT, I do chat with a number of people that have them, and a few institutional keepers of the species. I would highly recommend the Clemmys list on Yahoo Groups to learn more from people with first-hand experience. I've learned from their mistakes, which they freely share to better the care of the species.
Spotteds are not deep water turtles like the sliders, cooters, and terrapins. They tend to move slower than these other species, thus getting less of the food. They grow slower as well, which will compound this problem as time goes by. You might be watching to make sure that each turtles gets his or her fill at feeding time, but if life gets hectic, you might not be able to watch as closely. From talking with keepers, is seems that those with mixed collections tend to loose more spotteds for one reason or another. The truly infuriating part is to have one keeper loose an adult spotted a year for the last few years, and still not consider to do a necropsy to find out WHY he's loosing so many.
Also, I've heard of a number of spotted keepers complain of drowned spotteds when kept in deeper water. This is especially problematic when males and females are kept together in deep water.
Did these turtles come from the same breeder? I ask because, as mentioned earlier, spotteds are, IMO, too important to risk cross-contamination. We don't think too much of mixing sliders with cooters and others, but those turtles are relatively common, and the loss of a few individuals isn't important in the grand scheme of the species. Loosing individual spotted turtles now, though, could make an impact on the species survival. One here and one there eventually add up for a small species with limited range and specific requirements. FYI, do you have a gram scale so that you can chart their weight?
As for tail nipping, mud and musk turtles tend to be very nippy. They also prefer the water to be on the shallow side, unless they have something to use to climb up to the surface.
Katrina