Thanks, ashes2ashes, that's good to know. It opens up my stocking options.
I was also thinking about the floating idea. Plastic needlework grid floats (the kind people use for moss walls). You could cut some squares of the size you needed, put them in the bottom of a casserole dish, and pour the gel over them. Maybe if you got the thickness of the gel right, the plastic would keep it afloat, and when the fish were finished you'd reuse the grids.