So if our fish start dieing are the chances that testing the water will tell us why?
yes.
the tests will show any toxic levels of ammonia and nitrIte
no2 (both of which should be zero at all times)
and dangerouse levels of nitrAte
no3.
the pH should be ideal for the fish you have, so a pH of 6 for malawi cichlids is dangerouse
but a ph of 8 for them is great, a ph of 8 would be deadly for amazonian fish like neons or glowlights.
all of these water parameters can change as the tank matures and they can all be recified (if they go wrong)
just by doing a few partial water changes. also they can be kept stable by doing a small (10-20%) water change
every week or every other week for larger tanks.
remember fishkeeping is not just keeping fish, you are trying to keep an entire ecosystem and the water is the
main part of that system, the more you know about the fishes enviroment the better you will be able to keep them.