It didn't do anything bad to my tank. I have cut more since and have them in my big and small tank....I only have a few pieces left that I am waiting to add to the main tank. I had heard that adding too much rock at once would upset the tank. I'm not sure if that is true or not so I add a piece or two every week until I am finished. I only have 2 bigger pieces left and I will be done adding until I get more. I did notice that the piece I forgot to rinse left the water really cloudy and I was gonna do a water change until I realized that tiny pieces of rocks are sand right. It took several hours to get the water clear again.....but everyone survived it and I have added more cut rock since with no bad effects on the inhabitants.