scary
 

Maybe shore up the side that is drooping with 2by4's. I really dont think your slab would shift without you noticing some new cracking. I would check out those stands and make sure they are still structurally sound. that is where the problem is most likely located.

Jon

tank
 

put a level on it.It sounds like your tank is sinking.You have to move the tank and redo your floor.I have seen additions on houses sink and settle.If your tank isnt level look out it could exsplode on you.Dont wait fix it now