levels get over 100ppm your fish would be in danger. If you are using the downloadable calculator, there is a tab where you can change the limits to define what's too low and what's too high. The default values are very conservative.
is made up of mainly Calcium and Magnesium. These are nutrients that your plants need to grow. Generally these should be fairly stable, but you could see some change especially if you have a high snail population using the calcium to build their shells. Some rocks and substrates will buffer the GH
, causing it to rise. It can also change if your tap water parameters have changed.