Warning WIll Robinson, Danger Danger!
Here is the support skinny. All of the weight is being supported by the single 2by header framing the stairs. The weight on the floor joists is transfers the weight onto that header and the header on the opposite end. BUT, since the tank is above the end of the joists next to the stairs, only a small percentage of the wiehgt is actually being transferred to the wall with vertical support.
As an immediate measure, accurately measure the distance from the bottom of the header to the floor. cut a 4x4 approximately 1/4 inch longer than that measurement and tilt it into place with the bottom centered under the header approximately at the end of the tank...this doen't have to be exact but use a plumb bob to make sure the bottom end is directly under the header. drive the top end of the 4x4 under the header (do this inline with the header and it will slip right into place with a liberally applied 6 lb hammer)nto place under the area where the tank is. Go to the other end and repeat the process... measure the distance for number 2 AFTER number one is in place. Cut number two about 1/8 of an inch longerthan the measurement. Place the first 4 x 4 closest to the bottom of the stairs, the second back towards the top of the stairs.
Engineering is such a fine art, get the BIG hammer, Bubba