I agree with Thor on the Aquaclear filter, it is the best common HOB (hang on back) filter for that price on the market. There are a few that might be better, but cost more. like the Fluval C Series. The Marineland/Biowheel filters are good, I've used them. Generally, you have to constantly change the cartridges (which is what they want you to do (how they make money), and you loose bacteria every time you do this. I just got my first Aquaclear on a new tank I am setting up now. I love it. The pluses to it are you have more options to setting it up. It operates the same way a canister filter does, only smaller. Which is really a huge plus. The minuses are, even though when set up properly you won't have to do as much maintenance, when you do it will usually be more than just replacing a cartridge. The other is if you take full advantage of the flexibility to set it up in a way best suited for you, you will have to learn a bit more about how a filter operates (which again is actually a plus to me, but it isn't quite "plug & play").
The other thing I wanted to add, you can keep some fish in less water than the "recommendations" safely. Like a small/fancy goldfish in a 10gal. But it requires better/more filtration, and usually more work maintaining it. Lastly, if you want to understand how filtration makes the water non toxic to fish, it would be best to read up on what the nitrogen cycle actually is, as that is what turns the toxic ammonia (which the fish produce) into less toxic compounds. I'm sure Thor has an article here somewhere or a link to such info.
And lastly, the Petco "$1 a gallon" sale is good. But remember you are getting nothing but the tank. You don't get a hood/canopy, light, or anything else, so you will need to purchase all the separately. It may be cheaper to look into one of the kits, once you add all the costs up.
