I'm talking about an average house that has been already built as far as supporting a tank of up to 500g. Certainly it can be done with different materials like 4 by's or even steel and concrete. How many houses have steel reinforced concrete floors?
Well, let's start over in our consideration of what the typical house. Modern building codes require that every floor support a minimum of 40 pounds per square foot. 60 pounds per square foot is more typical, with 100 pounds per square foot being on the high end.
Next, consider a waterbed. That's one of those pieces of furnature that people begin to wonder if their house can support the weight of that piece of furnature. Well a typical king size water bed can weight around 2,000 pounds. But that weight is spread over an area of over 75 square feet. The net weight of the water bed is about 25 pounds per square feet. That's well below the 40 pounds per square feet standard, so any modern house should feel safe placing a waterbed in the bedroom.
Next, consider a 55 gallon tank. The tank is going to weight around 500 pounds or more. But it occupys only 4 square foot of floor space. That makes of a net weight of about 125 pounds per square foot. So even with just a "small" 55 gallon tank, you are already exceeding the load limit of an extreamly well constructed floor. Of course that just means that you can NOT fill your room with 55 gallon tanks. A single 55 gallon tank is not going to cause your floor to collapse. But in a 12ft x 15ft room, a single 55 gallon fish tank can represent about 10% of the total weight thr room can safely support.
So even with a simple 55 gallon tank, you have to begin thinking seriously about support. At a minimum, you are going to want the tank near a wall and not in the middle of the room, and you are going to want to ensure that the tank weight is spread across as many floor joists as possible.
But given the numbers I just reviewed regarding a 55 gallon tank, I would say that anything larger than a 55 gallon tank and you have to either consider placement of the tank VERY CAREFULLY, or you have to reinforce the house in some manner. That reinforcement can be as simple as installing sister floor joists to the existing joists, or it might require framing some sort of support under the floor.
Framing in a support is VERY doable if your house has either an unfinished basement on concrete, or has a crawl space over dirt. In the case of a crawl space, you can add supports the way you would for building a deck (4x4s on either a poured footer, or deck blocks). In the case of an unfinished basement, you just have to have the practical space to frame in something like a closet or storage space under the tank. Basically, you build a wall under the tank. Standard stud walls, especially if you frame with either 2x6 or doubled up 2x4s can hold LOTS of weight. The major limitation in wheither a floor can hold 40 or 100 pounds per square foot is the floor joists, not the walls. I would therefore claim that if a 15ft x 12ft room can hold as much as 60,000 pounds with the right joists, that means the walls can support about 1,000 pounds per linear foot. So a 4ft x 18in closet framed in a basement under a fish tank could support a tank upto around 10,000 pounds, and that would be equivilent to a 1,000 gallon fish tank.
So if you have the practical space in the basement to install reinforcement walls, the typical home owner is only limited to the size fish tank his budget can affort. Because when you start talking about fish tanks greater than 100 gallons, the cost of the tank is going to be more than the cost of building a support wall under it. So your only true limit is where you can build support walls, OR your limitation is going to be in the height of your fish tank. The taller the fishtank, the higher the pounds per square foot load is going to be.
So consider a 600 gallon tank. I've seen tanks about this size in commercial applications. For those that know about them, Guthrie's used to have salt water fish tanks in all their resturants. These tanks were about 6 feet long, 3 feet wide, and 4 feet deep. As 600 gallons, the weight would have been about 6,000 pounds, or 350 pounds per square foot. Given that a house wall can be made to support 1,000 pounds per linear foot, and such a tank has 18 linear foot around its base, a modern house can easily install a 600 gallon fish tank if they have the basement room to build a standard load baring wall under the tank.
Based on that last set of measurements, I would say that the only thing limiting the typical home owner is that he can't have a fish tank any TALLER than about 8 feet tall without having to resort to EXTREAM measures to support the tank... again, assuming there is the practical space in the basement to build a support wall.
