How about a fish tank jelly? Preferably use a (clean!) goldfish bowl, make the jelly up using lemonade and lots of fish sweets (you know fishy fish etc). You could start with green jelly then mix it with yellow then end up with yellow. You layer it up by putting in some jelly with a few fish sweets, leave it to set then add another layer etc. If you do it all at once the fish will all either float to the top or sink to the bottom, layering it means that you can have fish all the way through. You could put some of those rainbow drop things at the bottom to represent pebbles.