GCSE Biology BioenergeticsWatch
If this is testing the rate of photosynthesis depending on light level, it's pretty desperate and I'm not sure if it is one of the answers they're looking for but using the same volume of fresh water each time as well so that they're equally oxygenated. It's kind of more a fair test thing than reliable results.
Hope that's helpful. I'm not very confident with the 2nd one at all.