Goldfish Breath: How does a decrease in the dissolved oxygen level affect the breathing rate of goldfish?
(Excerpted from Experiment Central, published by U·X·L and available online in Science Resource Center.)
Purpose/Hypothesis
In this experiment, you will observe the breathing rate of goldfish as they swim in water with different levels of dissolved oxygen. [Note: It is recommended that you perform this experiment only if you already have access to an aquarium with four to six goldfish and only with the permission of a responsible adult.]
What Are the Variables?
Variables are anything that might affect the results of an experiment. Here are the main variables in this experiment:
- the health and size of all the goldfish
- the temperature and cleanliness of all the water
- the level of dissolved oxygen in the different containers of water
Level of Difficulty
Easy/moderate.
Materials Needed
- One 10-gallon (38-liter) or larger aquarium that has been set up for a month or longer and uses an air pump to constantly bubble air through the water (the aquarium may also include live plants, which add more dissolved oxygen to the water; other fish living in the aquarium will not affect the experiment, as long as they have been there for several weeks)
- One half-gallon (1.9-liter) container
- 4 to 6 small goldfish
- Dissolved oxygen test kit
- Stopwatch
- Fish net
- Red and blue colored pencils
- Goggles
- Rubber gloves
Approximate Budget
$15 to $20 for the test kit. (Ideally, you will be able to use an aquarium that is already set up at school or at home.)
Timetable
15 minutes to set up the small container; 20 minutes to check the dissolved oxygen levels and breathing rates every 2 hours for 6 hours.
How to Experiment Safely
Treat the goldfish gently; avoid putting them into water that is warmer or cooler than they are used to. Limit the duration of the test to no more than 8 to 10 hours. Wear goggles and gloves to protect your eyes and skin while you test the water because you will be using chemicals that can be dangerous. Have an adult help you complete the tests.
Step-by-Step Instructions
- If you have to purchase additional goldfish to conduct the experiment, place them in the aquarium and allow 24 hours for them to get used to the water. During this period, if the aquarium has a heater, turn it off and allow the water to reach air temperature. Make sure the air pump continues to work.
- Using water from the aquarium, fill the half-gallon container.
- Use the kit to test the dissolved oxygen level in the aquarium and in the half-gallon container. They should be the same at this point. On a graph similar to that illustrated, record the level from the aquarium in red and the level from the small container in blue.
- Use the net to catch half of the goldfish (2 or 3); put them in the smaller container.
- Use the stopwatch to measure how many times each goldfish breathes in 30 seconds. Each outward push of the gills is one breath. Average the breathing rates for the goldfish in the aquarium. Use the red pencil to record the average on a graph similar to that illustrated. Then average the breathing rates for the goldfish in the small container, and use the blue pencil to record that average on the graph.
- Wait 2 hours and retest the dissolved oxygen levels in both containers. Then average the breathing rates of the fish in each container. Record your findings.
- Repeat Step 6 after 4 hours and after 6 hours.
- At the end of the experiment, gently put the goldfish from the small container back into the aquarium. If you disconnected the aquarium heater, plug it back in.
Summary of Results
Study the dissolved oxygen levels on the first graph. What do you notice? Did the levels change in the aquarium? Did they change in the small container? If so, why?
Now compare the breathing rates of the two groups of fish, shown on the second graph. Notice whether the breathing rates changed as the levels of dissolved oxygen changed. How did the goldfish respond to any changes in the levels of dissolved oxygen? Was your hypothesis correct?
Troubleshooter's Guide
Problem: The dissolved oxygen level in the small container remained the same.
Possible cause: The fish were too small to affect the level during this time period. Try the experiment again, using bigger or more fish, putting them in a smaller container of water, or extending the time period for the testing to 8 or 10 hours.
Problem: The breathing rate of the fish in the aquarium and the container dropped.
Possible cause: The water temperature might have fallen enough to slow the body processes of the goldfish. If possible, move the aquarium and small container to a warmer spot. Or leave the aquarium heater plugged in and put a heater in the small container to keep the water at the same temperature as the aquarium--a difficult feat to accomplish.