I'm not going to get into a credential war with you... so let me address why I suggested what I did. Most small table top type aquariums come with some form of filtration, which I was assuming the OP had (and they did). Kept at room temperature without any heat, an aquarium will be in the low seventies. Check out any care sheet for my suggestions and the temp ranges will be between 65-75. You suggested the Betta, which most assuredly would need a heater. White Clouds would be the best choice for this setup IMO, far better than a Betta, which requires a much warmer temperature range.
To the OP, as with any type of aquarium, your best friend is going to be partial water changes. An undergravel filter is not entirely useless as it is a great medium for biological filtration. Might I also make another suggestion? A morph crayfish. They're beautiful, have a ton of "personality", and thrive in seemingly all types of conditions. I've currently got this beaut in about 2.5 gallons of water, and it's doing great.
You would have to upgrade to a slightly larger setup in the future, but you'll fall in love with it, and want to give it the best conditions. Whatever you do decide on, good luck and have fun!