Just thought of another one.
My first reef tank was a 29 gallon. I was 18 and thought I knew a lot more then I did. I had button polyps, xenia, and mushrooms. About $500 in livestock. I was told that if I add buffer it will make the xenia pulse more. So I kept adding without testing the water. When my corals started to melt I realized something was wrong. I stopped adding drops to the test vile when I got to 60. Needless to say a dKH of 60+ can really melt some corals. Fish survived though, must have been like swimming through concrete the water was so hard.