It is quite common for floors to sag at the middle of a joist span. If there is a load-bearing wall in the middle of the house, it is common for the the floor to slope down away from the middle, but to then slope back up towards the exterior walls. If the slope goes continually down from the center wall without going back up,then there may be some concern. If this is the case, it is possible that there has been settlement at the exterior walls, which is generally related to the footing and foundation. Since a concrete cistern filled with water weighs a lot, it may be that the extra weight has compressed the soil more in this area, leading to more settlement. Wall cracks are usually associated with this type of movement. Look to the links below as your first step in diagnosing the problem. Unfortunately, wood siding, such as that found on your house, can mask movement. You may need to focus in the visible portions of the foundation wall. Even if settlement has taken place, it's important to know if movement is ongoing. You should monitor any settlement cracks that you do find and try to determine if the movement has stopped, or if it is still progressing. Have a look at the links below for more information, but generally, sags are normal, but slopes are often related to settlement.