Take your time, research, read, think about it, reconsider, research some more, then go to the store.
green terror, oscar, jack dempsey, frontosa, convict, skat, var. puffers, mono, white tip shark, var. pleco, labs, neon tetra, cardinal tetra, discus, betta splendin, serpae tetra, mollies, pictus cat, otocinclus, blood fin tetra, cory cat, and anything else I forgot to mention.
To many to mention, all plant tanks running DIY Co2 systems.
fish don't need toys, get that crap outta the tank!
Keeping anemones in a reef tank can be a challenge if you are not fully prepared.
Cultivating a nano tank can be an exciting challenge but unless you choose the right fish for your tank, you may be in trouble.
When you see signs of stress in your fish, you can then take steps to identify the source of that stress and then to resolve it before it becomes a major issue.