craftgrrl | Suggestions needed/welcome

I'm trying my hand at resin molding.... wait, let me back up:

I'm fervently excited about decorating my fish tank. Far more excited than one rightly should be. But I'm finding that many of the things that I want to put in there aren't in the aisle at PetsMart or the local aquarium depot, and otherwise hard to find on the small scale needed to fit in the tank. So I've decided to create my own, by casting them out of resin. I've watched tutorials on the Internet about casting fake gems and such, which are only one sided, but I want to create three-dimensional pieces. My first idea was to carve what I wanted to create out of wax, coat with sculpty clay, and place in the oven to both harden the clay and melt the wax (which would escape through holes in the bottom of the clay: voila, three-dimensional one-use mold. Then I'd just pour in the resin compound through the holes and let it harden.

The problem with above method, so far, is carving the wax. I've taken candles and melted them down, poured them into a slab to harden, and then tried to carve, but it always breaks.

So. Any suggestions? Is the another, proven way to cast (that would be cost effective)? Or a way to make my method work?