Instant Rotifers are shipped live in volume from 1 to 500 million. This is the protocol we use for counting the rotifers and ensuring that you receive the number of rotifers that you are ordering:
We start with a 20ml sample from the middle of the culture tank where there is good circulation (to be sure you get a representative sample, stir the culture before taking the sample).
We add 10 drops of vinegar to immobilize the rotifers in a relaxed (not contracted) state, mix, and while the rotifers are still well-suspended, pipette 1 ml from the 20 ml sample onto a Sedgewick-Rafter counting slide, which is etched with a grid of 50 x 20 = 1000 squares.
We usually count the rotifers in two horizontal rows of squares (2 rows of 50 = 100 squares) near the middle of the slide and multiply by ten to get the number of rotifers per ml.
Your count must include a total of at least about 100 rotifers to ensure that the count is statistically reliable, so it may be necessary to count more squares. The rotifer counts should be repeatable to within 10% and are often within 5% of each other.