A new tank shouldn't had cracked so easily. It's a very serious problem for a new tank to crack.
What is the dimension of your 300 liter tank (width, height, depth)?
What is the thickness of the glass (sides and bottom glass)?
Did you put any styrofoam or rubber mat under the tank in case of uneven surface on the top surface of the cabinet?
Lastly, can you post a photo of your tank?
Sorry, for the many questions.
I'm trying to see whether if there is any bad designs with the tank that might have caused it to crack.