You will need to shim the tank so that the entire bottom is supported. Just putting a board across the front will put stress points into the stand. Those stands are fine but need to be completely supported.
Personally, in this situation... I would build a wood frame where the inner dimensions are the size of the base of the stand. Lay down plastic inside it. Then fill the frame with self leveling compound. When it hardens it would create a level platform for the stand to sit on. I'd either make the frame ascetically pleasing or remove it and paint the platform so it blends in...
Why is the floor leaning like that?