I agree with a GH and KH test kit. However they tend to be more important in a planted tank. If you live in an agricultural area I'd also suggest always keeping fresh activated carbon in your tank. Well water doesn't go through as many treatments as city water so it helps takes anything out that it might have picked up. I have a whole house carbon filter, but if that goes bad without knowing and I have no carbon in my tanks, all my fish die within 2 days. Just a bit of a warning.
Typically it's not actually well water that causes problems for fish but water softeners. Fish do well in my well water (given it's been through a carbon treatment), but plants and inverts on the hand can't survive because of how softened my water is. Water softeners take out what plants, inverts, and fish need (but is bad for pipes and appliances) and replaces it with salt or potassium salt which in turn kills the plants and inverts.
Through the years I've went through many battles and tons of testing and research with my water. For my planted tanks I have a reverse osmosis unit that I have to use and then add in only the minerals that I want in.
Typically it's not actually well water that causes problems for fish but water softeners. Fish do well in my well water (given it's been through a carbon treatment), but plants and inverts on the hand can't survive because of how softened my water is. Water softeners take out what plants, inverts, and fish need (but is bad for pipes and appliances) and replaces it with salt or potassium salt which in turn kills the plants and inverts.
Through the years I've went through many battles and tons of testing and research with my water. For my planted tanks I have a reverse osmosis unit that I have to use and then add in only the minerals that I want in.