First off, Elvis, the desert wonder tortoise is alive and active! Now, I am building an outdoor pond for (what I am planning for) one or two FEMALE red-eared sliders. The pond will be incorporated into my desert tortoise garden yet the two breeds will not be able to see each other, so in essence will be able to cohabitate without really cohabitating (Does that make any sense?) To my question, the pond is fairly large (10' x 6' Average) and at its deepest point 3'. It will be lined with EPDM with gradual shelves staircasing up the exterior walls. I will have larger boulders and rocks along the staircases. My question is, should I ballast the bottom of the pond with a sand, river pebbles or gravel? Or just leave the EPDM exposed?