Hey, how you doing? I love nano tanks, but the water parameters are very hard to keep stable. When you say nano tanks, I assume you are saying 5 gallons and under. I am currently running a 2.5 gallon with driftwood and tons and tons and tons of java moss. The two inhabitants, blue dwarf crayfish and a pea puffer, love it. Bettas would be a nice centerpiece too. Do you have a filter? And a heater? There are also cold water fishes that don't require a heater, but selection is small, the only small ones I can think of are White Cloud Mountain Minnows. If you had a heater, the variety and selection is much bigger! For any small tank, you could do a shrimp colony, very nice and breed easily. When keeping shrimp, water quality needs to be impeccable and pristine. Scarlet Badis are a tiny fish, very beautiful and perch like that would be a great centerpiece. They require live and frozen foods, and they can be hard to get, depending on where you live. The possibilities are endless! Go out there and do some research, I'm sure you'll find something!