I don't know why so many people think you can't put fish in a 5 gallon tank.
I currently have two 8-inch fantail goldfish in the 5 gallon!!
And there is absolutely nothing wrong with them. They have enough room to move around and they are perfectly healthy. I've had them in there for almost a full year, since they were about 1 and a half inches, and they do not just languish around. They are fine and perfectly happy. My neighbor is getting a garden pond so I am donating the goldfish to him within a week or two.
I also have a 10 gallon tank with:
3 WhiteCloud Mountain Minnows- 1 and half inches
3 Glow-light Tetras- 1 and half inches
2 Rummy-nose Tetras- 2 inches
3 One-lined Pencil Fish- 2 inches
1 Spotted Dojo Loach- 8 inch
1 Clown Pleco- 2 inch
1 Talking Catfish (Acanthodoras spinosissimus)- 3 inch
1 African Dwarf Frog- 2 and half inch
8 Ghost Shrimp
Up and running for over 2 months
NO PROBLEMS
I don't care what anyone says about this. THIS TANK IS NOT CROWDED!!!