Water Quality - Unit 3
Purpose: The purpose of this experiment is to help
the students appreciate the response of goldfish to different levels of
dissolved oxygen in the water.
Materials for the class:
- 40 liter aquarium
- Bag of ice
- Microwave, stove or hotplate (each group will need to heat the water
in one of the beakers)
- 1- roll of plastic wrap
Materials for each group:
- 3 goldfish OR 9 daphnia magna (large water fleas)
- 3 - 1000mL beakers (Pyrex or other heat treated glass)
- 1-Large plastic tub (~ 30 cm long x 30 cm wide x 15 cm deep)
- 1- Electric aquarium air bubbler
- Dissolved oxygen measuring kit
- Marker and masking tape
Note: Read all of the instructions before proceeding!!
Depending upon the availability of goldfish, you may elect
to use daphnia magna water fleas. If you decide to do so, substitute 3
water fleas for 1 fish in the experiment. Also, the students will be heating
water in a beaker and placing it in a water bath to reduce the amount of
oxygen that is present in the water. You may elect to boil the water ahead
of time to reduce the possibility of the students being burned when they
are handling boiling water. It is important to make sure that you use
heat-resistant glass beakers (such as Pyrex) at least for those beakers
that will be heated (1 per group).
Suggestion: When designing this experiment we had in mind
that you, the teachers, would divide the class into groups. This experiment
is designed to be flexible given each teacherís available supply of fish,
beakers, etc. So, you may modify our setup given your number of students
and available resources. However, step 13 requires that there be at least
4 members in a group (3 to observe and 1 to time.)
1. To begin with, have the students fill all three of the beakers with
water from the aquarium that the goldfish are in. Instruct them to fill
each one with 750 mL (milliliters) of water.
2. With masking tape and a marker, have the students label one beaker
"High Dissolved Oxygen",
one beaker "Low Dissolved Oxygen",
and the third "Control".
3. Instruct the students to fill the plastic tub with ice, until it
is almost full. Then add water to it until the water is about 2
centimeters below the rim of the tub. This will serve as the water bath that will
be used to cool the Low Dissolved Oxygen
beaker in step 7.
Preparation of the Beakers:
4. Have the students place the electric aquarium air bubbler in the
beaker labeled High Dissolved Oxygen.
Let it bubble for at least 10 minutes.
Note: Be sure not to remove the aquarium bubbler until immediately
before you place the fish in the water.
5. Instruct the students to set the Control
beaker aside. Use the thermometer to check the temperature of the
water in this beaker. Be careful not to agitate the water; disturbing the water
will increase the amount of dissolved oxygen in the water.
6. Have the students heat the beaker that is labeled Low
Dissolved Oxygen using the microwave, stove or hotplate.
Heat the water until it boils and allow it to boil continuously for
at least 5 minutes.
7. After boiling the water, have the students place the beaker (Low
Dissolved Oxygen) into the ice-water
bath. The bath should be in a large container that can hold the
beaker. Put in as much ice as possible and add water, filling
it to a level just below the height of the beaker.
8. Instruct the students to place a thermometer in the water and immediately
cover the beaker with plastic wrap. Explain to them that they
need to push the plastic wrap down inside the beaker so that it is
touching the surface of the water. There should not be any air space between
the water and plastic wrap. Have the students remove the beaker from the ice-water
bath when the temperature has dropped to approximately the same
temperature as the water in the Control
beaker (~24-26 ļC).
Note: Make sure they stir the water gently to get an accurate
measurement of the temperature. Stirring the water very gently
will help to distribute the temperature evenly throughout the solution.
If they stir the water too briskly, they will reintroduce oxygen into
Determination of Dissolved Oxygen:
9. Help the students measure the level of dissolved oxygen of the
water in each beaker using the measurement kit.
Note: Be sure to carefully follow the directions included with
the water quality kit.
- 10. Have the students make predictions on the behavior of each goldfish
in the three different dissolved oxygen levels. The students should
record their predictions
in the Student Journal Activity.
11. Have the students take three of the fish currently in the aquarium
and place goldfish into each of the beakers.
- Note: While doing this, try to avoid agitating the
water. Keep the plastic wrap over the Low
Dissolved Oxygen beaker at all times.
Make sure that the air bubbler in the High
Dissolved Oxygen beaker is removed.
12. Allow the fish to be undisturbed for 5 minutes
to acclimate to the different environments. Remind the students to be sure to note the time when
the fish were placed into the beakers.
13. After 5 minutes have
elapsed, have the students make observations on the behavior of the
Select one of the following physical movements to monitor in every goldfish:
- swishing of tail
- flexing of mouth
- flexing of gills
- Each of these movements are normally performed by fish. Count the number of times the goldfish makes
this movement over the period of 1 minute.
- Have the students record their observations in the Student Journal
Note: Counting the number of times that the
fish swishes its tail is probably the easiest measurement to make.
14. Have students compare their results with what they predicted.
The results will be based on any behavioral changes observed.
Are the results the same as the predictions or are they different?
Is their behavior different than from the control group?
15. As a class, discuss the results. If any effect was (or wasn't)
seen, discuss what happened and how the fish were affected. If there
was a difference in their behavior, did everyone's experiment provide
the same results? If there were no observed effects, why did this happen?
What could be done to change the experiment that would possibly produce