How to oxygenate the aquarium?
Goldfish consume lots of of oxygen. The air pump (aerator) is a small and inexpensive device that promotes gas exchanges by raising the bubbles to the surface, and consequently improves the oxygenation of the water.
The air pump is not normally required during the day in a well planted tank and with a clear movement on the surface, and may be detrimental to plant growth, as it allows CO2 to escape.
However, if your aquarium is sparsely planted or not planted at all, if no movement at the surface is visible (stagnant water) or if your fish become sluggish or often surface and gasp for air, the air pump is an essential way of increasing the water oxygenation. Overcrowded tanks (competition for oxygen between your fish) or tanks with new tank syndrome (nitrite prevents fish from absorbing oxygen properly) often don’t have enough oxygen available.
An air pump is useful as well in a ‘hospital tank’, when a fish is sick and contagious and needs to be isolated.
In addition, during the night, plants compete with fish for oxygen: connecting an air pump at night is a way to allow better oxygenation of the water.
The air pump is also helpful in the event of a heatwave because, all other things being equal, warmer water contains less oxygen than colder water (low saturation rate).
The air pump connects to an air stone or diffuser (preferably get a fine air stone) with a tube (get a silicone tube, as ordinary plastic tubes become rigid and unusable quickly).
For safety reasons (to avoid accidental back flow of water to the pump in the event of a power failure, for example), it is recommended to place the air pump above the water level: this is not always practical, so it is possible to purchase a check valve (or non-return valve).
Lastly, some basic filters, such as the box filter (see below) or some under-gravel filters need to be connected to an air pump to create the movement of water through the filter media.
Another common use of the air pump is to connect it to a box filter. This type of filter is nothing more or less than a plastic box with a grid on top (or sometimes on the sides). The water is passed through the filter media thanks to the rise of air bubbles generated by an air pump. The water is therefore both filtered and oxygenated. This super cheap filter (only a few dollars, plus the air pump cost) is best for small or quarantine tanks for example.