My Coreys have been acting weird for two weeks. If not been as active as they normally are but they eating fine. At first it was just the two Julie Corey's acting this way. Now my peppered Cory is acting sick as well. They are sitting on the bottom a lot kind of tilting to the side. I can't see any sign of disease or illness but they are getting worse. What should I do? Tank parameters are normal and tank is cycled. I've had to Coreys for four months but I introduced a new guppy a month ago. No other fish are sick.