This is not pure rain water that he collects. It is collected from obviously contaminated sources and all those creatures actually come from the soil, not the rain. Let some tap water (that is guaranteed to have no life in it) sit in a jar for a few days to get rid of chlorine, and then add a little potting soil or some dirt and leaf litter in it and soon you'll have tones of algae, planaria, nematodes and protozoa swimming around. With rain water usually it's the same, it normally contains only very little sand and dirt and perhaps a few bacteria that it picked up in the air here and there, nothing else.
Hmm.... I was expecting the video to be about what lived in rain water and only rain water. Instead the video is about whats living in puddles of collected rain water. Still very cool but a bit misleading.
Misleading title. This is not rainwater, it is a puddle. There are all sorts of creatures living in the dirt.