Thanks everyone. Yeah, depending on where the Neon Tetras are, it seems like there is 11 of them or about 15 of them. I did the math and measured the inside angles of the tank then converted the cubic inches to gallons and I found out that it was 30.13 gallons.
I should send a picture of the tank of how it looks like now to the previous owner. I still have his e-mail address...