The point is I can't afford anything over $100 for the complete system. I want to do something to help the ecosystem and the best thing I can do right now is aerate. This means that either I can say "Oh, I can't afford a quality pump, so I won't aerate," or I can say, "there must be something I can do for under $100, anything is better than nothing."

Can anyone suggest tubing and a diffuser?

Thanks!