Since it's an older house, it probably was built level, & just settled over time. The tank is probably around 55 U.S. gallons, making it around 700 pounds with the stand, water, & gravel. You want to make sure ot is set up perpendicular to the floor joists, not paralell. This will offer more suport.
Shim the stand to level between the floor & stand. Fill it half way & check level again, you may have to drain & adjust the shims at this point with an older floor, as they tend to give a little with the added weight. It may take a couple of times to get it level when it is full.
Tanks are designed to hold water in a level position, the more they are out of level, the more uneven the stress placed on seams. This could cause a leak in the future.
Tolak