If you bought this plane new, and the tank was full of fuel, then you bought a used plane. The RTF version comes with the tank installed, but no fuel since it's a hazardous material and has different shipping requirements.

I'd have somebody with a lot of experience look this plane over really good to make sure it's not messed up, especially the engine or better yet return it for another that has not been used.

For balancing, you may have to add weight to the tail. I have an Alpha 60 RTF that required 2-3/4 oz of lead on the tail to balance properly - and my wing repair didn't cause the nose heavy condition (bought it with damaged wing hence the repair before it was even assembled).

Hogflyer