I can see a great deal of difficulties with it, among them problems of:
-Contamination of different types of algae.
-Different nutrient supplies in the water you are growing algae on.
-Actual measurement of the extent of algae growth.
-Statistical issues (you would need to conduct the experiment many times in order to get a meaningful result).
-Controlling for other factors (e.g. type of container, etc.).
I guess that it depends on what your aim is.
If you are trying to demonstrate that algae accumulates at different rates on different surfaces, then it's a good project.
If, however, you want to know which one accumulates fastest and which accumulates the slowest,then you will have to be VERY careful about your procedure.
I would suggest filling up your tanks with tap water,and cleaning (thoroughly!) the surfaces that you want the algae to grow on before adding them to the tanks. You need to fill the tanks up at the same time (tap water might have different amounts of stuff in it at different times). You also need to store your tanks all in the same place, with the same light, etc.
For the algae, I would go to a local pond or drain and take a sample, and measure out a volume of water from your sample (say, a tablespoon, or the like), and pour an equal volume into each tank. Then you wait for a period of time for your algae to grow and see what happens.
Good luck!

That sounds like a great idea. There are many types of algae, but the way to find the one that grows best in tanks is probably to talk to people at a local fish store and see what tends to build up in their tanks or their customers' tanks.

You might also ask around your school to see who has a tank that needs cleaning. The owner will probably be happy to donate their algae to you.

If you requested people's used jars or 2L bottles, you will probably have plenty to use as tanks. You will also have to make sure you use the right kind of water. Tap water will probably have too much chlorine. You can ask the people at the fish store, or your friends with fish, what they use to treat their water.

Think about what things must be identical in each tank and which things you want to change. Remember that if you change two things at once, you won't know which thing had an effect on the algae. I would advise wearing plastic or latex gloves to protect yourself from contaminants. Also, wash your hands and equipment well.

Great ideas!

However, you need to think carefully about how you want to quantify the AMOUNT of algae growth on these different surfaces.

If you put in a piece of plastic and measure the mass of it before and afterwards, that might work if you have a very sensitive balance (say it can measure to 0.01g or so). Measuring sand would be more difficult! You might want to change your project slightly to look at differences in algae growth in, say
1) different light levels (high/low), and/or
2) different temperatures (warmer/colder), and/or
3) different nutrient levels (if you have a way to add nutrients to water).

Generally, algae growth should be faster in higher light, warmer water, and water that has more nutrients.

That sounds like a great idea for a project, and you can definitely do this in aquarium tanks.

Testing growth rates on various materials shouldn't be a problem; testing the effect of depth will be a little trickier in tanks. Perhaps you can get at the depth question via a more indirect route light intensity. As you go deeper in the water, the light intensity will decrease. Thus, you could set up multiple tanks with different light exposure and compare growth rates on the same materials under those different conditions.
Id recommend setting up the tanks away from a window or other uncontrollable light sources so that you can regulate the amount of light they get. And try to keep all your different pieces of material at the same distance from the light source. It would be ideal if you could use tanks with the long fluorescent bulbs in the lids. That way the light is shining down equally to all areas of the tanks. Then you could modify the lids to block some of the light coming down, or buy lower watt bulbs to create dimmer conditions in the tanks with less light.
You should probably get several blocks of each material youll be using. First of all, you need to have enough pieces to put copies of every material in each of your different light treatments. But secondly, you might also want to consider putting two or three pieces of the same material in each tank, placing them in different areas (corners, edges, middle).
This should help control for any variation in growth that might occur in different locations, because it may be the case that certain areas of the tanks will grow algae faster than others.
Now the big question: where do you get the algae?
Believe it or not, the easiest way to deal with this is by going to your local aquarium shop or a pet store that sells fish. Talk to the employees there. They deal with algae in their fish tanks all the time (although to them its a hassle!) so theyll have a lot of anecdotal information about the stuff. And theyd probably be happy to supply you with a source of algae for your project.
Anything should work, but ideally a floating plant that you could rest at the surface of your tanks would be best. That way its far away from your blocks of material. An algae-covered rock would work too, but then youd have to put it on the bottom with your materials, and youd have the problem of the rock being closer to some of the blocks than others. And take into consideration what type of water youre using - if youre doing the project in freshwater tanks, ask for a freshwater algae sample.
When you get the algae into your tanks, youll want to help stimulate it to grow. The algae will do it on its own, but a way to encourage it is by leaving the tank in the dark for 18-24 hours on the first day. The dark will stress the algae a little bit. Then when you turn the light on, the algae will be quick to take advantage of the good light conditions to reproduce. After the first day, you can then begin to simulate "normal" day/night light conditions. Try to turn off the light for 8 hours every night. Pretty soon you should have algae growing in your tanks and you can start comparing the growth rate and abundance on your different materials.
Good luck with your project!!