Tropical fish require well-oxygenated water to breathe and thrive. The optimal oxygen level for most tropical fish species is between 5 and 8 milligrams per liter (mg/L). Some fish, such as tetras and barbs, may require slightly higher levels, while others, like loaches and catfish, can tolerate slightly lower levels around 4–5 mg/L. Oxygen levels below 4 mg/L are considered too low and can cause stress, disease, and death.
To maintain adequate oxygen levels, aquariums should have filtration and aeration systems such as air pumps with airstones or powerheads to agitate the water surface. Live plants also help oxygenate the water. Regular monitoring with test kits or oxygen meters is recommended.