steve wrote:What's the white stuff? It should be fine with all the cleaning you've done.
White stuff is leftover oyster shells that I couldn't break off with a putty knife. I was able to get all but the most deeply imbedded portions off, and have paid particular attention to them when scrubbing. Thanks so much for the help you've provided. It's great to have a place to ask questions where I can receive knowledgeable answers I can have confidence in.
Since my turtles are so small yet, and the river rocks are pretty large, I'm going to have to wait a short time to replace their substrate, but will definitely take your advice and replace it with a sand bottom. Thanks again, Steve! Off to research what kind of sand to buy, and how to clean it.