Alright, I will try to help out the best I can on this.
My father is a home builder, has been for almost 30 years now. I have been helping in the construction of new homes since I was 4, so I have been in the industry for a long time too. Over the past 7 years or so, I have been studying the more technical aspects of home building since my brother and I are interested in the real estate markets around here. So I do know about the load a house can handle.
Bottom line is, if your house is UP TO CODES and was built after, well probably the 70's. You can guarantee the live load of your floor will handle 10,000 pounds before structural integrity begins to compromise. The only thing I can think of to make the live load larger would be to replace floor joist with steel ones or pour a concrete floor. Both of these can handle substantially larger loads that would be difficult to challenge. BUT you MUST consider EVERYTHING on the floor in the room you with to place a large tank in. Furniture, pianos, bookcases, kitchen cabinets, ding room tables, etc. all of these things add up big time. usually floors have about 3000 pounds of stuff put on top of them, from what I have experienced, so a tank up to about 600 gallons would be safe on the first floor of any newer home. This tank would need to be long so it could span across several joist and be placed up against a load bearing wall, if you cannot tell which one that is, then place it against a central wall that runs to the roof of the entire house, ie through the second floor, or along an exterior wall, either the front or back of the house.
Why can't i place my 600 gallon tank along a side exterior wall? Because it would fall through the floor. Joist in a house are run from front to back on the first floor and side to side on the second. So placing a large tank on a side exterior wall would stress only one joist whereas placing the tank on either the front or back exterior walls will stretch the load amongst several joist and thus reduce the load and strengthen the support.
Why against a load bearing wall? Because a load bearing wall is not supported by the floor, instead the floor is supported by it. If you go under your house or in your basement, take a look at everything. You will notice cinder block pillars below certain spots of the floor, these are supports for central load bearing walls. The columns can take an immense load as well as the rest of your foundation. Typically any wall resting directly on the foundation is considered load bearing, though there are some other instances where this is not true. Non load bearing or false walls are simply in place for looks or as room dividers and nothing more. Placing a tank against one of these is like placing a dump truck in the middle of your living room, it gets ugly.
Which brings my next point DO NOT PLACE A LARGE TANK IN THE MIDDLE OF A ROOM! The center of the floors are typically unsupported, which means placing the same tank from a wall to the middle of the room could mean compromising your structure. thankfully newer building codes are causing pillars to be placed in the middle of large floors, but they are different from CITY to CITY, so your house might be able to handle the tank, and a similar house 10 miles away in the next city could not.
If you already have a large tank in a place that is not properly supported or if you are worried, there are warning signs to look for. The first thing you might notice is a bounce in the floor when you walk by. This means your structure is compromising, not quite compromised. This could also be due to termite or other pest damage, water damage, but if these problems are not apparent then it is probably your tank. A depression in the floor be it carpet or wood is caused by the tank, bu is no reason to worry about your structure, this just means you need better flooring next time. A sag in the wall, cracks in the ceiling, sudden leaks in the roof, and foundation cracks all mean the load in the home is not being distributed correctly. This doesn't mean to evacuate, your house shouldn't collapse suddenly, just means call an engineer and a trusted contractor, and probably your bank. These repairs can be very expensive. If you have a lot of people over lets say, 20 people at 100 pounds each, and you notice some floor bounce, do not panic. This weight is distributed entirely differently and should not cause problems, but you should make sure everyone doesn't get into a small group and start jumping up and down simultaneously.
I highly suggest if you plan on placing a large tank i.e. above 200 gallons on any floor of a house made with wood to have a STRUCTURAL ENGINEER come and inspect the floor, foundation, and load of the house and to take their advice seriously. It will be the best 100 bucks you ever spent. IF you plan on putting a tank on a second or higher floor of a house that is more thank 60 gallons, get an engineer, not a home inspector, not a codes inspector, not a contractor, AN ENGINEER. These guys come armed to the teeth with logarithms and calculus equations that are just beyond our level of expertise. Besides that you aren't paying these guys to do the work, so engineers will give an honest opinion, not a sells pitch. Home inspectors are worthless, I am sorry, but I have yet to meet one who wasn't full of poo.
sorry for the long post, but this should be enough information to answer everyone's question, I will be watching the thread so anymore and I will be glad to answer the best I can.
the house was built in the 30's i think and the tank spans across about 4 or 5 joists, its a 125. and i have a 55 sump underneath it, so I think the extra support could help
