Its actually a sponge. My teacher had taught me this because I had taken her geckos over the weekend and she (and now I with my geckos) uses a sponge in the water bowl to help prevent drowning of the geckos.
I thought it was a sponge, but it is always good to ask. A sponge is a brilliant way to prevent water bowl related accidents. My daughter lost a very nice little peacock chick because she put water into a dixie cup with no sponge. He upended into the cup, upside down of course, and drowned. It was very sad, so I am glad you have measures in place to prevent such occurrences.
Five females all together, eh. Why so many? Are you guys thinking of breeding them?