When critter sensor is placed in a room with the Tropical variant of Pacu/Pacu Fry/Fry Egg, the sensor seems to fluctuate rapidly. The minimum value observed seems to be the number of Normal variants and the highest value observed seems to be the sum of both the Normal and Tropical variants.
Another bug report submitted describes the same issue: Critter Sensor not correctly detecting Tropical Pacu Fish Fry and Fish - Oxygen Not Included - Klei Entertainment Forums