While I understand your mom's fear of collapsing floors, I have a 20 long on the main floor (above the basement) of our 143 year old house. No problems whatsoever.
Mine is low maintenance, too, but I do weekly water changes out of habit (but only 3-4 gallons each week). I have 9 green neon tetras, 3 kuhlis loaches (hold overs from when the tank had an Asian theme), 1 clown pleco (almost 15 years old now!), and one Bolivian ram.
Everyone gets along great...I wouldn't mind replacing the kuhlis with some dwarf cory cats, but the kuhlis are cool in their own right and as shown by the 15 year old clown pleco...I tend to keep fish until they drop.
Eric