Being that you live in california I know from personal experience that the quality and thickness of the slab in the garage really depends on the builder. I am assuming the garage was built after the home which is why you don't know the year. If this is the case it is a plus because someone would take more care when building a garage for themselves than a builder slapping them up. I would dig next to the foundation to determine the thickness. Just remember the perimeter of the slab is usually raised a bit for the walls so find the thickness from the top of the slab.
The other thing you need to think about is settling. Just because the floor has no cracks in it now does not mean it can hold a few tons of water with out cracking.
I would say do it and use a liner with 2" foam all around it. I would frame it with 2x4's. This is assuming it is for catfish and no viewing window.