Aquarium Nitrogen Cycle

We as fish keepers should understand that fish will often adjust to poor electrolyte and calcium (& other necessary Redox reducing elements), however this does not prove this is best for the health of your fish anymore than how fish will often adjust to aquariums that are crowded with infrequent water changes (which is also not good for long term fish health).

Aquariums that are overdue for a water change, with high bio load, lack mineral/electrolyte replenishment, or simply initially start out with incorrect chemistry often results in inadequate mineral and carbonate chemistry necessary for optimum bio function of the aquatic inhabitants (fish and invertebrates).
Correction of this problem via water changes and/or addition of mineral or carbonate supplements may even show initial stress until the inhabitants adjust to the improved water quality.

Often aquarists in both fresh and saltwater worry too much about pH while ignoring the importance of mineral cations/electrolytes (found in part via GH), and KH (more correctly identified as alkalinity, as it is in SW). As for pH, stability is more important than the actual pH number in freshwater.
In marine aquariums the actual pH number is of higher importance than in most FW aquariums (due to the fact here are much lower natural fluctuations in pH in natural bodies of saltwater), however even in saltwater a correct alkalinity (KH) is a must for proper buffering of marine pH and as well elements such as Calcium, Magnesium and Chloride are also very important to a healthy saltwater aquarium as you will read further in this article.

Popular but outdated aquarium traditions state that only certain fish need water high in electrolytes and calcium, this is only partly true.
It is noteworthy that the Fish Body (as an organism) is alkaline by nature and is constantly attacked by free radicals which are usually acidic.
While freshwater fish absorb needed H2O (saltwater drink constantly), the need for these electrolytes and calcium for some fish (such as Amazon River fish) is lower than some, however Proper Osmotic function still requires that calcium and other electrolytes be present in the aquatic environment as well as foods.