Goldfish are great subjects for science fair projects because they are inexpensive and easy to care for. Alternatively, you can use shrimp, crabs, snails or even water fleas. Choose an independent variable in its environment that you are able to change, and then decide what you will be testing to analyze changes that result. For example, you can change the plant density in the tank, the tank size and population size in each tank, the temperature or salinity of the water, or you could add various pollutants to the water, such as petroleum or antifreeze. The dependent variable you should test for could then be the growth or death rate of the animals.
The ocean is subject to many environmental changes, either naturally occurring or pollution-induced. Some great project ideas can focus on how these factors affect ocean currents. One thing you can do is to create your own model in your home, filling a glass dish with water, to test certain variables and their effects. To be able to study the velocity of currents, sprinkle in some herbs that are visible and lightweight enough to float with the current. Prop the pan up and carefully experiment with zero, one or several lit candles beneath it, observing the convection currents that result from the heat. Calculate the velocity at which the herbs get carried and compare them to the water temperature.
The oceans are affected more and more by pollution and the increase in atmospheric carbon dioxide. One effect of this growing problem is that it is increasing the oceans' acidity. Because this is such an important issue in today's world, the topic makes for a terrific science fair project. To gain some background knowledge, do some research on pH, how fossil fuels affect CO2 levels and how acidosis can harm marine animals as a result. Then design a project to model the effects of pH in a marine environment. For example, you can collect some crushed shells and separate them into several jars of seawater. Then add varying amounts of vinegar to each jar in order to have incremental pH levels. Record the appearance and mass of the crushed shells before and after one month of letting them sit in the jars, and analyze what effect the pH has.