6 Causes of Low Oxygen and Ways to Increase Oxygen in a Fish Tank If you notice low oxygen in your fish tank, there are some easy solutions. By Shirlie Sharpe Shirlie Sharpe Shirlie Sharpe is an aquatic expert and writer with over three decades of experience keeping and raising ornamental fish. She has consulted with The Minnesota Zoo and the National Aquarium. Shirlie has also authored a book on setting up aquariums. Learn more about The Spruce Pets' Editorial Process Updated on 01/16/26 Reviewed by Nick Saint-Erne Reviewed by Nick Saint-Erne Dr. Nick Saint-Erne, DVM, is a highly accomplished veterinarian and writer who has treated zoo animals and exotic pets for more than 35 years. He has worked with the Centers for Disease Control and Prevention to improve pet store animal care among other endeavors. Dr. Saint-Erne is part of The Spruce Pets' veterinary review board. Learn more about The Spruce Pets' Veterinary Review Board Credit: The Spruce / Alison Czinkota Key Takeaways Ensure proper tank maintenance, avoid overcrowding, and monitor fish behavior to prevent low oxygen levels.Increase water movement and perform water changes immediately if fish gasp at the surface due to low oxygen.Factors like temperature, stagnant water, waste buildup, and plant activity can impact oxygen levels in aquariums. Low oxygen levels are rarely a problem if an aquarium is well maintained and not overstocked. However, if fish are gasping at the surface of the water, that should set off a red flag, prompting further investigation. The average aquarium fish will not survive more than a day in an oxygen depleted tank. Here is what to look for and how to remedy the problem of low oxygen. How Much Oxygen Do Fish Need? The optimal amount of oxygen in a fish tank varies according to the species of fish you have and the atmospheric pressure of your home's location (the higher the altitude, the less oxygen water can hold). Fortunately, you don't need to calculate an exact amount based on these parameters. If your fish appear active and happy, and none are gulping air from the surface, your tank's oxygen level is sufficient If you would like to measure the exact parts-per-million (ppm) of dissolved oxygen in your aquarium, purchase a portable dissolved oxygen meter. A generally safe oxygen level for all freshwater fish is 8.3 ppm. Signs of Low Oxygen Unfortunately, there is no flashing light or blaring alarm that goes off when dissolved oxygen levels drop too low in an aquarium. Aside from actually testing the water for dissolved oxygen, the only indication of trouble will be the behavior of your fish. Fish will initially react to lower oxygen levels by moving around less. They will swim less vigorously and even eat less often. As oxygen levels drop further, the fish may show labored breathing and more rapid gill movements as they desperately attempt to get enough oxygen from the water by passing more water over their gills. Eventually, fish will begin gasping at the surface of the water. This surface breathing should not be confused with fish feeding at the surface or fish species that normally "breathe" at the surface, such as labyrinth fish. 6 Causes of Low Oxygen and How to Fix It If your fish are gasping at the surface of the water, take emergency action to immediately increase the tank's oxygen: Perform a large water change (as much as 50 percent of the tank's volume). Then increase the water movement by adding a powerhead, airstones, or even an additional filter. These measures will introduce more oxygen to the aquarium, buying some time to address the underlying cause. Overcrowding Overcrowding is the number one reason for low oxygen in an aquarium. The rule of thumb for stocking a tank is one inch of fish per one or two gallons of water. High Water Temperature Higher temperature water cannot hold as much oxygen as cooler water. If the temperature in your tank is too high, turn your heater down (or off, temporarily). Performing a water change with water in the 65 to 70 degree Fahrenheit range will introduce fresh oxygen and safely lower the water temperature. Additionally, a few ice cubes placed in a zip-close bag can be placed in the tank to help drop the water temperature. Be sure not to lower the temperature below the ideal temperature range for your fish. Water Movement Stagnant water has low oxygen, so aquarium water must be agitated or moved regularly to maintain adequate oxygen. Filters help increase oxygen because they move water, so make sure your current filter is operating at full capacity. A clogged filter must be cleaned or replaced. In addition, you can install a spray bar on the outlet of the filter or add air stones to the tank. Dirty Tank Accumulated fish waste and algae overgrowth can sap oxygen from an aquarium as bacteria work to break down the excess organic matter. A thorough tank cleaning to remove debris from the gravel and tank walls will boost oxygen. Live Plants Sometimes, live plants can reduce oxygen in an aquarium. During the day, they absorb carbon dioxide (CO2) and release oxygen (O2). However, at night, this process reverses, causing plants and algae to consume oxygen. Increasing light exposure during the day can help raise oxygen levels. When the tank is dark, the process reverses, and your plants—including algae—will consume oxygen. Adding a light or increasing the lighting duration during the day can help raise oxygen levels. Chemicals Some chemicals used to treat fish diseases or modify water pH can also impact the oxygen level. If you need to use a chemical additive, err on the side of safety by increasing the tank's water circulation for the duration of treatment so that it maintains sufficient oxygen. FAQ What is the optimal oxygen level for most freshwater fish tanks? Most freshwater fish thrive at an oxygen level of around 8.3 ppm. How can I tell if my fish tank has low oxygen levels? Fish will move around less, swim less vigorously, eat less often, exhibit labored breathing, and eventually gasp at the surface when oxygen levels are low. How does temperature affect oxygen levels in an aquarium? Higher temperature water cannot hold as much oxygen as cooler water, so it's essential to monitor and adjust the temperature to maintain healthy oxygen levels. Explore more: Fish & Aquariums Aquarium Care & Maintenance Sources The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Aquarium Oxygen: The Essential Ingredient to a Healthy Aquarium. Fish Lab.