The microcosm method is used in the study of population dynamics of predation and prey and in the research of of food chains, which provides an opportunity for modeling and reconstruction of relationships in populations and biocenoses in nature. The aim of this paper is to investigate the interspecies and intraspecies relationships of competition, predation, and symbiosis in freshwater microcosms, between green hydra, freshwater turbellarians, large water fleas, and isolated endosymbiotic algae. The aim is also to study the share of extracurricular activities in the field of science among primary school students regarding the attractive content that can be offered within extracurricular activities in primary school and the attractiveness of attending an experiment. Experiment was performed in five replicates at two different temperatures (25 °C with a photoperiod of 8 h day/16 h night and at 13, 5 °C in the dark) and with fed and hungry animals, including control replicates. It has been shown that the hydra is the most interesting and most important competitor and predator and that the algae create a web by participating in the predation, while the turbellarians create a camouflage coloration as a result of predation. The method of anonymous survey results show that the prevalence of extracurricular activities in the field of science among the surveyed students is very low, and that they are attracted to the contents that include practical work in biology, especially those using a microscope, so the proven research can be a good basis for extracurricular activities in primary school.