Justice wrote:
What thickness of acrilic would I need to make a tank thats 45x12x24,
holding at least 500 lbs.

It kind of depends.
The number varies dramatically, depending on if the acrylic is structural,
or other stuff (steel/...) is being used to support the acrylic.
Braces across the top can also dramatically change the numbers.
And as acrylic is not cheap, do you want all sides acrylic, or only one?