Well, you do have to concede that it's your parents house you're living in.
They pay the mortgage and the electric and water bills, and well...it's their house! What you have to do is compromise with them and make them realize that one larger tank will look alot nicer than a bunch of overcrowded ones scattered about. Then, you have to get hold of yourself and curb your
MTS urges until you move out!
For some reason, some people believe that tanks larger than 10 gallons are inherently unstable and will spontaneously burst open.
I wouldn't use this argument on your parents, but 10 gallons of water on the floor is almost as much of a PITA as 30 gallons on the floor. If they're Ok with a 10
gal, I see no reason why they should be opposed to replacing it with a 30
gal. Just reassure them that you don't plan on upgrading to a 55 while you're still living under their roof.