I have a 3 gal pico, its been going for over a year now with no issues. I have only ever had nano tanks and never an issue if you don't overstock it.
In mine I have: Ricordeas, green star polyps, zoas, anthelia, 1 yellow clown goby, 2 sexy shrimp, 1 pom pom crab, 1 cleaner shrimp, 2 snails, 1...