Bad Couple Days

We have been having a bad couple days with the lab tank.  Today, we took out 8 dead ones and found at least 5 more spinners that are probably going to perish soon.  Trying to figure out what is the matter.  One fatality was examined under the microscope and large abcess-like growth was found on it's head.  On a good note, the fish that were transferred to the room tank are feeding well and seem to have adjusted well.