Normal floor live loading on houses is 40 pounds per square foot (psf) (2015 IRC) (not what was mentioned above). In upstairs bedrooms it is 30 psf (2015 IRC) and less in attics. Floors are also designed to distribute 300 pounds on a single square foot. The 40 psf live loading is assumed over the entire tributary area of each floor joist, which is usually the length of the joist times the joist spacing (usually 16 or 24 inches). A rough estimate for tank weight is 10 pounds per gallon (include sump water); water is a little under, but you have glass and rocks. So if you have a 220 gallon tank with 4 adjustable feet, you are supporting 2200 pounds + stand weight or 550+ pounds per adjustable foot, more than floors on joists are designed to carry. A 12' joist at a 24" spacing would be designed to support a distributed live load of 80 pounds per linear foot (plf) or 480 pounds over a 6 foot stretch. A tank oriented parallel to the joists would have 2 point loads of 550 pounds or 1100 over this same area. The point load would be excessive, and the distributed load would be excessive. The most notable effect early on an overloaded floor would be a floor that deflects (sags) and is very springy when you walk on it. Over time, it will continue to sag and possibly break. Of note, the 40 pounds per square foot is not considered to be constant, but the max distributed load expected with people and furniture on the floor. Constant loads are make joists permanently deflect and fail faster. And, there are building limits on floor deflection that vary depending on the type of load. Large tanks can exceed these. And placing tanks at the center of the span is the worst location.
If you orient the tank perpendicular to the joists and near the wall or supporting beam, most of the weight will be supported by the foundation through shear and not joist bending, usually. And there will be much less deflection of the joists. It is possible to overload joist hangers if the joist are hung on the side of beams or rim boards or overload ledgers nails if ledgers support joists.
I could go on more as why not to have point loads over 300 pounds, but suffice it to say that if a foot is over a joist, the plywood is not supporting any load. If you are on a plywood panel between joists at a plywood seam, less weight can be supported. A stand on a floor with floor joists should have enough leveling feet (and vertical supports above) that no leveling foot carries over 300 pounds, regardless of the foot capacity. And if you have no feet, have enough vertical members and shim at them so you have less than 300 lbs at each shim. In fact, most shims can not support much weight. Wood shims have grains in the wrong direction and plastic ones are not usually solid. So the more shims the better. Shims in the building industry are used to keep doors and windows vertical, not support weight.
On a cement floor, the loading would be less pronounced, if the builders properly packed the dirt under the slab, or if it was disturbed. Many do not pack correctly. Usually the ground underneath a slab can support 1500 pounds psf or more, but not always. This should be you preferred tank location. More load per foot is reasonable.
If you have any doubt, get a licensed professional engineer from your state to evaluate your fish tank location and provide you with options as needed. Don't go by the advise of someone that it worked fine for. Each house is different. Each location is different.
Having said all the above, does anyone have examples of floor failures under their tanks? The failure could be a tank failure as well if it is not evenly supported. It could be a very bouncy floor (as indicated by water slopping around in your tank as you walk by. It could be a hole in your floor sheathing.
