Do i need to test for magnesium or calcium in a fish only saltwater tank?
The above is correct, but you need to also test for Alkalinity, since PH is a direct result of Alkalinity. Ph should be tested early morning and at night after lights off. In the morning PH is at its lowest and at night it is at its highest. You need to know your range to make sure it is within safe limits. You do NOT need to test for magnesium or calcium in a fish only system. They are not being used up by anything. If you had live rock you would have to test for magnesium because it would use magnesium for things like coraline algae, but in fish only there is no need to test for Mag or Calcium.
