Based on the data provided, the number of rotifers alone reaches a peak of 4,200 on Day 16 and then remains stable at that number until Day 18. This indicates that 4,200 rotifers is the maximum number that the flask can support in the absence of water fleas, showing the flask’s carrying capacity for rotifers.
Therefore, the flask's carrying capacity for rotifers in the absence of water fleas is 4,200.