OK. I got it. That's funny about goldfish. My 9" female will eat a dozen fish in about 10 minutes

I'm not sure if they're 'goldfish' per se - are yours specifically goldfish? Are they large in that they are too large for the turtle to eat? I'm afraid to put any fish in the tank that are 'nice' because she WILL try to eat them - and even getting a 'nibble' is enough to kill almost any fish. I wanted to put a big algae eater in there - but I'm pretty sure she'll kill it - am I wrong? My smaller male tends to eat fish more slowly. Six feeder fish will last him like a week or so. I'm not sure why but he's not quite as voracious as the larger female (they're about the same age). I really wish I could put fish in there - their tanks are so bare and boring.
I have the Manzanita wood I want to put back in. I was wondering if I could put in these giant sea shells I have had since we took a trip to the carribean years ago to a small island called Jost Van Dyke. But I'm afraid of what these shells might do to the water in terms of what they give off in terms of minerals and other substances that I'm not knowledgable about. After all, they were from a saltwater environment so they might have some impact on freshwater.... I guess I should post that question in the habitat section????