Better pictures and water parameters are needed. One possibility is your new fish has brought velvet which can kill quickly and show spots near death. You might need to get your fish in a hospital tank asap running with an airstone. If it's velvet, ich, or brooklynella, Copper Power or Cupramine will treat the bugs. I have had luck with using Hydroplex in a 10 minute saltwater dip to give quick, low stress relief. It does kill the parasites on them but you still need to do the hospital treatment too.
These medications cannot be used in your DT which would need to go fallow for weeks depending on the type of parasite. See stickie threads under the disease forum.