you're testing to see if photosynthesis can occur without the presents of light you are using a small potted plant,aluminum foil, paper clip and iodine.
To make this a koi pond will not be easy with this set up - they wouldn't survive if it freezes solidThe only thing I can think of (and this probably won't work as it would be a huge expense) is to build some sort of retaining wall around the perimeter to raise the level of the pond and somehow seal the wall to help hold the water inside? Concrete blocks and lots of mortar might work? Not sure if the swampy land would even make this possible, but that's all i can think ofI'd hate to have such a liability issue in my yard without doing something to protect anyone from falling in, that's for surewow Good luck
No starch produced in the areas of the plant leaf covered by the foil, when tested with iodineLeaves will appear to be stained black by the iodine in those areas exposed to light indicating starch production as a result of photosynthesis.
