Lisa, first off, thank you!! There's no bigger compliment to receive than to have someone tell you your work is inspiring

I am going to say no on the feather rock because it is ridiculously sharp. Every time I work in this tank, I walk away with bloody hands, I don't even want to imagine what it would do to a frog's sensitive skin. If you really want black, light rock, I would find a place that sells the large red lava rock, paint it with krylon fusion black spray paint and then clear coat it with fusion clear. Same effect, but much safer.