aquarium oxygen level

Signs Of Low Oxygen In The Aquarium

Goldfish Care: Low Oxygen in the Aquarium

Caution: When your fish are at the surface gulping air,  your red flag should be going up prompting your immediate attention.

What are the Symptoms of Low Oxygen?

There are a few things that you should look for if you suspect your goldfish are not getting enough oxygen. 

  • Fish are lethargic and not moving around very much.
  • Fish are not eating.
  • Breathing is labored with rapid movement of the gills.
  • Fish are staying near the surface of the water. If all of your fish are gasping for air at the surface, consider it a fish emergency!

What Causes Low Oxygen in Aquariums?

There are generally two causes of depleted oxygen in your aquarium water.

1. Overcrowding of Goldfish Aquarium  (#1 Causes of Low Oxygen)

Having too many fish in the aquarium. Overcrowding causes the water quality to break down quickly because of waste produced by all of the fish.

Additionally, the more fish there are, the quicker the oxygen becomes depleted.

A goldfish’s lifespan is remarkably up to twenty years. Additionally goldfish size increases as they grow.

With these things in mind, preventing overcrowding is crucial for a successful goldfish tank.

2. Poor Water Quality

Not performing regular maintenance on your aquarium causes the water quality to drop significantly.

Other Possible Causes of Low Oxygen

If you have thinned out the overcrowded fish, it’s time to set up regular maintenance. 

This keeps the water quality high, maintains healthy fish and is less work on you.

When having to act quickly for sick fish, poor water quality, etc., the time consumed in correcting it could have been saved with proper maintenance.

Filtration System

Your filtration system is a lifeline for your fish. Not only does your filtration system filter impurities and waste in the water, but it also aerates.

If your filtration system is working overtime for a tank that is too large, you will have problems with water quality dropping.

Lastly, check your filtration for possible clogging and clean if needed.

Goldfish Water Temperature

To maintain optimum water quality, goldfish water temperature must be maintained at the correct temperature.

An aquarium that is too warm promotes growth of algae and algae depletes oxygen and causes poor water quality.

Cold water holds more oxygen than warm water. Your lighting tends to warm the water.

It is ideal to have your lights on for up to eight hours and off the remaining time.

To cool the water, you can remove the top cover of the aquarium and place a nearby fan pointed in the direction of the aquarium to blow air over the water. 

Monitor your fish as some fish may  jump out of the aquarium.

Fish Tank Aerator

Stagnant water contains low levels of oxygen whereas moving water aerates the water bringing in more oxygen.


Live plants in an aquarium contribute to low levels of oxygen in the water. Strangely, if you have the aquarium lighting on, the plants will produce oxygen. However, if the light is off, the plants will use oxygen.


Before adding any chemical or treatments to your aquarium water, read the label first. Some will affect the levels of oxygen in the water.

How do I correct low levels of oxygen?

1. Test the Aquarium Water

The first thing you should do is test your aquarium water for oxygen levels.

The ideal method of testing your aquarium water is with a portable dissolved oxygen meter.

The meter will need to be calibrated.

Once done, place the probe into the water.

The results will appear on the digital display. These meters tend to be expensive, and some fish owners prefer to rely on a cheaper method such as chemically testing the water.

You can find both the oxygen meter and chemical test kit at your local pet supplies store.

2. Do a Large Water Change

Performing a 50% water change will help the situation. When doing this, increase the water movement with a powerhead, air stones or place an additional filter in the aquarium temporarily.

The fresh water and increased water movement adds additional oxygen to the water.

3. Correct Overcrowding

We generally want lots of color and movement in our aquariums. Goldfish tank size should not be too small for your goldfish and tank mates.

Unfortunately, because of this, we tend to put too many fish in one aquarium. Once we do this, the decline in water quality begins the moment the “one fish too many” goes in and starts to swim around.

The fundamental rule of keeping your aquarium from becoming overcrowded:

1 Fish (1” fish)  for every 1 gallon of water.

 If you have too many fish, it’s time to thin them out and place the extra fish in another aquarium.

How do goldfish breathe?

Fish require oxygen just as any living creature does. They have a special process to extract oxygen from water. The oxygen that a fish extracts from water is eight hundred times denser than air.

To achieve the required levels of oxygen while in the water, fish use both the mouth and the covers of the gills to form a pump-like movement. This moves water over surfaces within the gills that absorb gases.

Blood that is passing through the gills is pumped in reverse order from the water coming in.

This process increases oxygen absorption. The oxygen then enters the blood and is  transported through the fish’s body.

Unlike a human, fish cannot live out of water.

There are some species of fish that can be without water for a very short period of time such as the walking catfish; however, most fish will not survive.

Once a fish is removed from water, their gills collapse as a result of water not flowing through them.

Unfortunately, this places stress on their ability to breathe and they die.

Your fish thank you for taking preventative measures to ensure they have a healthy aquarium and being quick to act when in distress.