DIY is great!
Get a couple of glass diffusers. I'm 13 too, and I got two shipped to me for $35, so it's not very expensive. The rest of the stuff is cheap. For a tank your size,
Equipment/Ingredients:
2x large juice bottle (about 4-5l if possible)
2x Check Valve (1 for each bottle)
Sachet of yeast
White Sugar
Water
Tomato sauce/vegemite/marmite
Baking Soda
My method (for 1 bottle):
Boil water. Add 2cups suger to a large glass jug. Then add around 3/4 a liter of boiling water, and stir to dissolve. Top up with very cold water so its warmish. Pour through a funnel into a bottle. Add yeast, 1/3 teaspoon baking soda, and a squirt of tamato sauce or vegemite etc. Put a lid on the bottle and shake furiously! Then top up the bottle with warm water and leave about 10cm from the top.
How to make a DIY system:
Get a bottle with a lid. Poke a hole in the lid (I get my dad to use an electric drill!). Put an airline joint in the hole. It must fit tightly. Then silicon it to the lid. Once dry, cut 20cm of airline and attatch to the joint. Then add a check valve, and to that add enough airline to reach into your tank, leaving about 7-10cm from the bottom. Attatch glass diffuser, and leave it for a couple of hours. The bottle should have built up enough pressure, and you will be able to see little bubbles coming from the diffuser.
Hope that helps
Joe