I have had that happen with pods contaminating cultures. The copepods are natural predators of rotifers. Its a rule of nature, where there are copepods, there are no rotifers; and where there are rotifers, there are no copepods. do you actually see rots in there? They should be really small dots suspended in the water. a good way to look at anything in cultures without a microscope is to put some in a drink glass and go in a dark room, then light a lighter on the opposite side and look through. you will see them much easier that way.