I found some rocks at the park near my house that I'd like to use to scape my 55g. Ive heard that you can use vinegar to see if it fizzes or not, than it should be safe for the tank. I also saw on youtube that using muriatic acid will get a better reaction. Has anyone ever used it to test out their rocks? If I post a picture of the rocks I found, would anyone be able to tell me if they're safe to put in my tank?