Common hystrix as low as 75 - 100 depend on size, true hystrix 135 - 175.I know that they aren't very deep but i will put a net over it. I was thinking about putting in a pond liner, some play sand, this filterhttp://www.drsfostersmith.com/product/prod_display.cfm?c=3578+3579+3580+16744&pcatid=16744. what type of ray could i have? I was thinking about a hystrix. how much do they usually run for?