An aquarium as an ecosystem

The word ecosystem defines the interactions that take place between the biological and non-biological elements within a defined space. The biological component of the ecosystem is called the biocenosis and represents all its living organisms; the non-biological part represents all its abiotic components (i.e. gas, minerals, elements in general) and is called the biotope.

Interactions, both internally and externally, occur and develop between the biocenosis and the biotope, and it is these interactions which define the ecosystem.

An aquarium can therefore be described as a closed artificial ecosystem in which fish and plants are able to find a habitat where they can grow and develop in a healthy and balanced way. Each aquarium must therefore be consciously designed, developed and managed to facilitate the establishment of the virtuous cycles that occur spontaneously in nature and give rise to the interactions.

Water: the importance of analysis

The main parameters that need to be considered and monitored, with regard to a freshwater aquarium, are:

  • pH
  • hardness
  • ammonia
  • nitrites
  • nitrates
  • phosphates

pH is the measure of the water's acidity. It is measured on a scale ranging from 1 to 14. Low values (up to 7) correspond to acidic waters.

A pH value equal to 7 indicates neutrality, that is, a balance between the acidic components and the basic component (for the latter, the index varies from 7 to 14).

For freshwater aquariums, the ideal pH varies greatly depending on the type of plants and fish it supports, but ranges approximately from 5.5 to over 8.0. Most tropical fish prefer medium or slightly acidic values. However, even when faced with a (non-sudden) change in this value, fish are able to adapt without difficulty, so much so that, in nature, the same species can be found in aquatic ecosystems with very different values.

A pH check is recommended at every partial water change.

Carbonate hardness is measured in KH; its function in the aquarium is to stabilise the pH of the water by opposing any sudden variations in it. A KH value between 4 and 8 is generally suitable for most freshwater aquariums; values below 4 expose the tank to the risk of sudden changes in pH. For marine aquariums, KH values range from 8 to 10. The KH value also affects the amount of CO2 dissolved in the water and its assimilation by the plants. By knowing the KH and pH values of your aquarium, you can work out the amount of dissolved CO2, thanks to the relationships that regulate these three amounts (see the table below).

Title: CO2 concentrations
Yellow: CO2 concentrations potentially dangerous for fish
Green: CO2 concentrations considered ideal for plant growth
White: Low CO2 concentrations


Ammonia is one of the products of protein metabolism of fish: the proteins ingested with food are hydrolysed in the gut and broken down into amino acids. The amino acids are then transported by the blood to the cells, where they are used for protein synthesis. Those not used directly are broken down to produce energy; during this process, ammonia if formed, which, being highly toxic, is excreted directly into the water. It is found in two forms: ammonium, with a relatively low level of toxicity, and ammonia, which is extremely toxic. The concentration of ammonia/ammonium in the aquarium, with the exclusion of the set-up period, should ideally be zero. The presence of ammonia/ammonium – even at low concentrations – indicates poor biological filtration.

We recommend performing the Askoll NH3/NH4 Test once a week.

Nitrates, NO3, are the final product of ammonia processing performed by filter bacteria and therefore tend to accumulate in the aquarium. Nitrates are a source of nitrogen for plants, which absorb them. However, except in the case of aquariums which are very rich in plants but have few fish, the rate of production is higher than the rate of absorption, and nitrates accumulate in the water. Although nitrates are relatively low in toxicity and well tolerated, their presence stimulates the proliferation of algae, so their concentration should not exceed 10 mg/l in marine aquariums and 20 mg/l in freshwater aquariums. A high concentration of nitrates causes stress to fish.

The phosphates accumulated in the water derive in part from the processing of fish excrement, but can be contained in a variable quantity even in tap water. They accumulate slowly, but, if in excessive quantities, cause the proliferation of weed algae (for which they are an excellent fertilizer), but not very pleasant from an aesthetic point of view. It's worth doing a PO4 test.

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