If it's a glass 300, I've used a piano moving company (I provided industrial suction grips - the type used for moving granite counter tops and the like) and a local moving company (again, I provided the grips). Neither would assume responsibility for the glass, mind you, but both did manage to get the aquarium moved when it needed to be. At about 450lbs empty, and a little awkward in size and such, I was more than happy to hire on some folks that could handle it. If it's acrylic, you're a little better off as it shouldn't weigh quite as much and should be easier to maneuver.
As for the current contents - if you're planning on moving the rock, coral, fish, etc you're probably going to be on your own. Plan the aquarium shut down out in detail, try to locate a local reef club or sympathetic pet shop you've been frequenting and see if they can hold/house livestock for you and either ship to you new location after you're set up or hold for you to pick up and travel with. Either are doable.
Take your plans, number out the steps/sequence of events, and then overlay on your move timing. Double and then triple-check to make sure you have all necessary containers, places to put them, plan for setting up and getting it running at the new location, and you're all set.
Now, after all of this hyperbole, if you're just moving the empty aquarium, stand, and assorted hardware, stick with the first paragraph above.
Cheers,
Ray