Regarding the fish: everyone makes good points above but neglects the most important detail: all these "goldfish" that kids bring home from camp, carnivals, and that are used in centerpieces are generally not goldfish at all; they're minnows. Goldfish (actually part of the carp family) look completely different, grow
very large, and yes, output an immense amount of shmutz. These need a real tank to survive.
Minnows on the other hand are sold as "feeder fish" at ten for a dollar, to be used as food by other fish. They already sit in filth in the pet store, and have a very short life expectancy, no matter how well you take care of them. In this particular case, the idea of blowing in with a straw is a good one and may prolong their life a bit. Not because you're introducing oxygen into the water (you exhale carbon dioxide, not oxygen...), but because the bubbles disturb the water surface, which is where the oxygen exchange takes place. Taking a fork and splashing the surface of the water around will accomplish the same thing.
Change 50% of the water once a week, and put in some sort of substrate to help keep the ammonia levels in check. Read at least a basic article on the nitrogen cycle, you'll learn a great deal.
And most importantly -
do not overfeed! The number one leading cause of death in fish is overfeeding, which wrecks the nitrogen cycle. Feet them real fish food once a day (not matzoh meal!), and never more than they could finish in 1-2 minutes.
This is a goldfish:
...and these are minnows: