Not quite nature but they are animals. We have kept fish on and off for over 50 years and recently decided to reinstall this 25-year-old 60L octagonal tank. I have been running it for a month empty of fish to establish the nitrogen cycle and allow beneficial bacteria to colonise. Been testing weekly and today it was set to go. Have two goldfish (a Blackmoor and a Veil-tail) and a Betta (Siamese fighter) who tends to hide most of the time. Being 8-sided it can produce double images but fluked a few that are OK.