You are correct that it is a declared weed in Tasmania, but not in other states/territories, and a permitted organism in WA see https://www.agric.wa.gov.au/organisms/85206
The following link has some good information about Ceratophyllum demersum:
- Hornwort (Ceratophyllum demersum) is a free-floating, submerged, rootless, leafy, annual or perennial freshwater herb, reproducing vegetatively and by seed.
- It occurs in sheltered sites in stagnant or slowly moving water in ponds, dams, streams and reservoirs.
- Hornwort is native to Australia, occurring in all states except Tasmania.
- In Australia, Hornwort rarely causes problems when it is in balance with the surrounding ecosystem and can be beneficial. However, when environmental change occurs, the plant becomes weedy and has a negative effect on stream flow, interferes with navigation, fishing and hydro-electric output.
- Herbicides provide the best means of controlling the growth of Hornwort.[i]
When the ecosystem is in balance, plants like Ceratophyllum demersum and Hydrilla verticillata do not generally cause a problem, but when there is an influx of nutrients or other changes occur that alter that balance, it can overgrow and cause problems.
Therefore, as you have said, it is permitted in WA so you can use that one. Of course, as you are aware of its potential as a weed, responsible management of it would include destroying any remaining plant material at the end of the activity to ensure that it is not allowed to enter any waterways where it does not already exist.
It seems that the plants that have the most potential to be a weed and cause environmental problems, are the best oxygen producers to demonstrate photosynthesis in science activities! Science ASSIST would welcome any suggestions for plants that may suit this activity.
[i] http://www.environment.gov.au/cgi-bin/biodiversity/invasive/weeds/weedde... https://creativecommons.org/licenses/by/4.0/ © Commonwealth of Australia 2013.