An aquarium develops permanently - it is not static. Some substances enriches by and by, others get to less. Those parameters you can measure with tests or electronical devices are easy to control. For some values there is technique to control, for example nitrate and phosphate. Other parameters like trace elements you cannot monitor. To be sure that these substances are in the right concentration the only way is to work with additives.
Most of the substances in an aquarium are unkown and you cannot measure them: lots of organic compounds belong to that group. To avert danger of too less or too much a regularly water change is necessary. We recommend a water change of 1% per week for sea water tanks and 5% per week for fresh water. Without a regularly water change the biological system will collaps: it will take months or some years - but it will.
Products for mineral supply for the animals: calcium, magnesium, carbonate hardness, strontium, iodine, iron, trace elements
In addition to the inorganic supply, the addition of organic compounds may not be neglected.
Products for solving problems in the aquarium: symptoms of poisoning, turbidity, bad supported corals.