It's good that you are finalizing the details of your experiment.
Do you have access to a spectrophotometer so you can measure the density of the green color at 550 nm? This would give you a quantitative measurement of your results. Or, do you have access to a microscope? I believe you could also count cells in a hemocytometer. However, measuring absorbance is much easier.
Here's a website that describes various methods for measuring algae:http://www.botany.wisc.edu/courses/bota ... ations.pdf
Do you have time to order a small culture of the algae and grow it in a stock container so you will have fresh and rapidly growing algae to start your experiment? Results for experiments like this depend on having cells that are at the same phase of growth, and algae that you receive by mail may take a while to recover and start growing. And you would only have to pay for the initial small culture.
What concentration of algaecide is recommended on the product label? You want to test a range of concentrations, perhaps 5-6 concentrations, that are below, at, and above the recommended concentration. Also, you need a control with no algaecide.
I recommend doing a pilot experiment with a control and one concentration of algaecide to test your method and find out about any experimental problems you will encounter. Then plan to do your experiment in duplicate, or better, triplicate. Using a higher number of replicates will allow you to do statistical analysis of the results, so more replicates would be better. However, each sample will require a lot of work.