I need help. I have an 80 gallon freshwater tank , take good care of everything, yet over the past four months?

we never had more than 11 fish in the tank at one time. The black bottom cleaners seem to die the most-3 in 4 weeks, a mono and three silver dollars died one after the other. We have plastic plants, changed the water (1/3) once in three months, don't overfeed. What can be the cause of the death? Thanks, Shelli