# An aquarium

It seems that the only things doing anything in my aquarium are the fish. They swim and feed. They breathe using their gills. There must be more going on!
There's some pond weed there and some green algae. If they are living, and they are, they must be doing something - why would the pet shop say that the plants were good for the fish, even if it doesn't eat them.

In fact, the plants are producing oxygen but apart from some bubbles, that is hard to see. Another way to 'see' oxygen is to use an oxygen sensor. This can measure the amount of oxygen in the water so when the oxygen level increases you can see this on a computer screen.
You'll know that plants need light to live so you would expect that the plants will be affected by day and night. So I've used a light sensor in this experiment on my aquarium where I'm trying to see what the plants are doing.

## What I did

I placed an oxygen sensor in an aquarium and set up a light sensor nearby. I connected these to my computer*, started it running and left it disturbed for several days.

*Actually rather than leave the computer running all the time, I used a data logger. This is a box which will automatically take readings from sensors over long periods of time.

Gratuitous special effect - you miss little

## The results

###### This is the graph of both sensors on the same axes.

You will find some of the following questions easier if you put these results into your data logging software. Click here to get the results, then Open it in your data logging program.

## Look at the first graph of the light level:

• How many days did I run this experiment for?
• How many troughs are there in the graph?
• What do you think the troughs in the graph correspond to?
• What time of day do you think the peaks in the graph correspond to?
• Why are the light peaks at different heights?

#### Extra:

• What is the time interval between each peak on the graph?
• Can you work out what time the experiment was started?

## Look at the third graph showing both oxygen and light level:

• Although the oxygen graph is noisy, how many peaks are there in it?
• Do the oxygen peaks match in any way with the light level peaks?
• Does the oxygen level reach a peak before or after the light level does? Why might this be?
• At what time of day do the plants produce the most oxygen? At what time of day do the plants produce the least oxygen? What does this tell you?
• What are the plants doing in the aquarium? How is this good for the fish?
• How can the fish breathe at night time?
• Why do you think the oxygen graph is 'noisy'?
• Get your software to smooth the graph. Does this still give a fair picture of the experiment?

### What you might find out

• Is more oxygen produced by plants on sunny days?
Does this cyclical (that is daily) pattern work the same in a pond?
Does this cyclical pattern work the same in a running stream?
What happens to the oxygen levels and the light levels when plants overgrow in a river?

Idea and results file by Laurence Rogers, University of Leicester.