Which available pondweed works for this and which supplier do you use?
it's the a level version where you measure the volume of gas in an expensive bit of kit.If it the usual bubble count from a cut stem, Hornwort works well, I grow my own at home and now hopefully in school.
Measuring the volume of gas means you can be more flexible with the pondweed than having to count bubbles (it doesn't matter if they are too small and fast to be counted individually) so pretty much anything that gives off bubbles from the cut end works. We also use hornwort from my pond.
plants alive had closed down.....There's Egeria densa from Blades.... bit more expensive than aquarium-type suppliers though.... : https://blades-bio.co.uk/shop/livin...ng-plants/egeria-densa-substitute-for-elodea/ We had success with Egeria densa for the GCSE bubble counting one (though we did get ours from plants alive before they closed down), so may be ok for yours....