In general, the bigger the sample size, the better.
One possibility would be to start with a large number of brine shirmp for just 3 pH levels at extreme values. (One at very high pH, one medium pH and one at very low pH). The results of this first run can then help you decide how many brine shirmp to use.
This assumes you have time to do more than one "run" for the experiment.
I did a google search and found this site on statistics and number of samples:
-Statistical significance and the number of analyses performed
-Why significance of a relation between variables depends on the size of the sample
-Example: "Baby boys to baby girls ratio"
-Why small relations can be proven significant only in large samples
-Can "no relation" be a significant result?
Good luck and let us know if you have more questions.