You can use them for various things...

You can buy a little cute pet turtle... and start an aquarium... she'll thank you for the rest of her 40 years.

BUT, regarding cannabis growing:
If you're going hydro, you can use the tanks. Of course! But, make sure you "black out" the sides, so no light gets to the inside (where the nutrient mix and the roots will reside).

Best Ninja,

-turtle420
.