You can house a single male betta in a 1/2 gallon jar, but that would be pretty cruel. Try for a 2-1/2 gallon aquarium you can get at PetSmart.

Any aquarium would be very heavy when filled with water.

You can house fish such as a couple of guppies, mollies, and a few neon tetra's in a 5 to 7 gallon tank. Make sure you get proper water conditioners and a good heater to keep your fish warm.

Hope this helps.

TammyPie