Lack of oxygen (hypoxia)
The word hypoxia comes from the Greek hupo: under, and oxus: oxygen.
If several fish show the following symptoms, low oxygen level or hypoxia can be the issue:
- Several fish become lethargic, feed little or not at all. They may also display fearful behavior, constantly rest on the gravel with clamped fins or else hang listlessly at the surface of the water
- In extreme cases, fish may have completely disordered movements, be agitated and desperate to jump out of the aquarium as if to escape from an invisible predator.
- Other common symptoms, in the instance of a nitrite surge, can include veined or red spots on tail or fins, frayed fins, pale color, bright red gills with possible bleeding and rapid / labored breathing.
- Simultaneous and unexplained death of several fish or unusual behavior of several fish at once.
Possible causes (can be more than one):
- aquarium too small or overpopulated
- a lack of movement at the water surface
- nitrite or ammonia spike
- goldfish bowl
- excessive heat in summer
- overdose of CO2
Start by testing for ammonia and nitrite (nitrite prevents fish from absorbing oxygen properly). If the water contains ammonia or nitrite, this is abnormal: regular partial water changes (every 2 or 3 days) should be made to dilute these toxins until the filter is sufficiently mature (read the nitrogen cycle). Finally, connect an air filter (aerator), which is essential in an unplanted aquarium and in the absence of a current at the surface.
Excessive heat in the summer can cause the available oxygen level in the water to drop. If this is the case, first turn off the lights on the tub, add a little cool water and install an air filter if you haven’t already done so.
Less frequently, a drop in pH can be the culprit (poorly maintained aquarium, overcrowded, water with very little mineralization). Check your pH, especially if your water is soft.