In wastewater treatment, rotifers are associated with "good" treatment where all major treatment parameters have been met. As a large - 0.5 - 2mm organism, the rotifer is interesting in that it is a multicellular creature with visible mouth, digestive system, and discrete organs. They can be attached to floc where they feed on bacteria and other organic particles.
In wastewater, we see rotifers appear under low F/M (Food/Microorganism Mass) or "older" sludge ages. There also has to be sufficient dissolved oxygen and bacteria floc. So, we have an indicator organisms found in "good" system health that is easily observed - what makes them a "less than ideal indicator organism?" I'll list the pros and cons below:
Easily observed Somewhat resistant initial spills
Seen with low F/M Usually indicate an older sludge than optimal