These Protozoan Ciliates are not actually parasites. They eat bacteria and use the host for transportation. This does irritate and create wounds on the host. These wounds create extra bacteria due to the decaying flesh, which the ciliates now eat, this now turn into a bacterial infection and so the cycle starts. These Ciliates are larger than parasitic protozoan. Ciliates are part of the biome in your tank, they are ever present and consume the detritus and waste. When tank conditions are not maintained the bacterial load can explode, then the ciliates population explode. The Freshwater or Saltwater fish aquarium are the perfect breeding ground.