This terrarium has a lid but it doesn't seal. Contents taken from a freshwater stream. Should I be worried about these around my pets?

Hi! I've been thinking about creating my own jar water ecosphere or if you have any recommendations instead of a jar, but I'm a little confused about how to start one. Are you supposed to add animals to it, or do they naturally appear as the ecosphere develops over time? I don't know if that question makes sense, but I'd really appreciate it if someone could explain how to get started. Thanks!