Paraben toxicity is a great topic for research! But now what organism can you use for testing parabens? One of my favorites for water pollution testing are water fleas, Daphnia magna
. They are multicellular, easy to grow and are quite sensitive to chemicals added to the water in which they live. If you have a pond or lake nearby, you can get wild Daphnia to use in your project. Otherwise you can buy them from Carolina Biologicals (https://www.carolina.com/living-organis ... aphnia_cat
Here's a reference that describes research on paraben toxicity using Daphnia and other organisms that you can read:https://link.springer.com/chapter/10.10 ... -3509-7_14
The dependent variables that you can use with Daphnia include their activity level, heart rate and egg production so you have some pretty good read-outs for data.
Another aquatic organism that is sensitive to pollution is algae. They are found in both fresh and salt water. Green algae like Chlorella in fresh water or diatoms in salt water are able to use sunlight or artificial light to photosynthesize and produce oxygen, which you can collect by displacement and measure the volume of. Parabens may affect their ability to photosynthesize and that could be serious in the ocean because most of our oxygen supply comes from phytoplankton in seawater. Carolina Biologicals sells these algae.
One other organism I'd like to suggest is a dinoflagellate. These are really neat ocean dwellers that have the ability to make light by bioluminescence. You can raise these unicellular plankton at home and under the right conditions they will glow with a pretty blue-green light. You could add a paraben to their water and see if it affects the light production.
There is one problem with testing parabens and that is what will you use as your test substance? For doing an experiment to test toxicity of a chemical, you need to be able to add that chemical alone to your target organism's environment. I don't know where you could get a paraben by itself except maybe from a company that uses them in products. Parabens are used in a lot of different things such as cosmetics and food, but if you add something that has a lot of ingredients you won't be able to say which one is causing an effect.
Do some more reading about parabens and toxicity testing and post again when you have decided on your hypothesis and plan for experiment to test it.