For those of you who know about fish

Okay 1st of all get a heater. Go to the Dr. Foster & Smith website and pick up a 100watt Visi-Therm Stealth heater for $16. Then from the same place buy a Emperor 280 filter for $31.

For that size aquarium I say get some Tetras and Dwarf Gouramis. The type of tetras and gouramis are up to you, but glo-lite, neon or cardinal tetras look nice. Perhaps take a shot at some rummy nose or emperor tetras if you want to try something a little more advanced.

Yes, a heater is a definite for any tropical fish. I do not recommend adding plecostomus into a coldwater tank. Some people think "oh, it's an algae eater, it will be fine." But they are tropical fishes and need the heat!
Goldfish actually do better in slightly cooler water; they are one fish that I would not recommend a heater for in normal room temperatures.

also, realize that the "one inch of fissh per gallon of water" typically only applies to very small, shoaling fish like some tetras and barbs. imagine a 14" peacock bass in a 20 gal tank.

I would go one further and tell you to ditch the undergravel filter and set up a nice susbstrate to support some live plants. There are quite a few that do well in low light, or you can spend a bit more money and get some compact fluorescents to grow the higher light plants. If you get a heater to keep the water around 75F you will find that you can have different plants and fish. Also, having healthy plants and not overfeeding your fish will keep algae blooms to a minimum. Once the plants are using up the nutrients the algae doesn't stand a chance.

By having live plants you will increase the ability of the tank to process waste from the fish, your fish will be less stressed and you will find that it is a nicer environment for all of you. I had a 20g long tank set up in my room and did not need to run a filter on it after it was established. It had a moderate fish load of tetras, catfish, danios and barbs. It's probably my favorite of all the tanks I've had over the years.