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Common examples of water parameters include pH, temperature, salinity, nitrogen wastes content, general hardness (GH) and carbonate hardness (KH).

Optimal water parameter values may differ significantly depending on the inhabiting species and the fact whether the aquarium is freshwater or marine.

However, regardless of the aquarium settings, it is generally accepted that the optimal measurable content of ammonia (NH₃) and nitrites (NO₂^-) should be 0, as they are both highly toxic to aquatic life. The only exceptional case where it's desired to have measurable amounts of ammonia and nitrites in the water is when one hasn't introduced any animals to the aquarium yet and attempts to start the nitrogen cycle going.

Nitrates (NO₃^-), on the other hand, aren't nearly as toxic. Generally, many fish species could tolerate suboptimal, raised levels of this substance and wouldn't display acute poisoning symptoms unless the nitrate content exceeds around 100 parts per million, but optimally the nitrate content in freshwater aquarium should not exceed 10 to 20 ppm. A prolonged exposure to slightly raised nitrate concentrations, exceeding 30 to 60 ppm, could lead to negative interference with fish' immune system, leading to diseases; it would also prevent them from fully thriving. Consequently, it is especially important to maintain low nitrate content in hospital tanks. What is more, elevated nitrate levels could cause an undesirable algae bloom since nitrogen (N) is often the limiting nutritient for plants, so generally it's wise to keep their concentration relatively low.

(source: article on nitrate levels in freshwater tanks at tfhmagazine.com).

For marine reef and coral aquariums, however, even a miniscule amount of nitrates could cause severe damage - for that reason, the general guideline for nitrate content it these types of settings tells it should be below 1 ppm.

(source: Ideal nitrate levels at fishlab.com).