Hi. I want to set up a nice 20 gallon. What is the rule regarding how
many fishes I should keep in the tank? Are there any differences
pros/cons between a regular 20 and a 20-long? What else will I need? Is
the hood with light required? What power filter would I need? Combos of
fish that cannot stay together (I know about bettas)? Can any tropicals
live with goldfish? Any other advice?

Thanks,

Lewis