At the end of the test period, count and record how many Daphnia in each dish have died. First, take a look at your control group. These individuals were not exposed to any toxic chemicals, and you might have expected all of them to survive. If a few died, that's OK with any type of living organism there is variability in health, life span, and sensitivity to environmental conditions. Perhaps the dissolved oxygen level dropped too low, or the individuals you started out with were not young and healthy.
Graphs are useful in interpreting the dose-response relationship.
From your graph, make an estimate of the LC50 for your experiment. Think about whether any of the Daphnia may have died for reasons other than poisoning by the chemical.