You should probably get permission from the landlord, but I've had aquariums in houses with crawl spaces. If the floor will hold heavy major appliances, it will probably hold an aquarium with no problem. In fact, most houses in the northern part of the US have basements, etc. Here in the desert southwest, most houses are built on concrete slabs, so heavy stuff on the floor is not an issue. In earthquake prone places like California, some houses are supported by steel beams rather than wooden ones.
Of course, the bottom line is the landlord's opinion, and if he/she doesn't permit large aquariums, maybe you could put them in storage and get several smaller aquariums. I like a 37 tall or a 25 tall for angelfish, and 20 gallon tanks are fine for most live bearing fish. If you get much below 15 gallons, you'll have to be very careful about maintenance chores, because small water volumes can change very quickly if they experience a problem.
Good luck, and please let us know what you decide.
