Bettas are the best choice for a small tank. However, bettas are aggressive and must be kept totally alone. White clouds or danios are also hardy enough for small space.
Bettas are best for smaller tanks, they don't even require filtration,though it will keep the water from stagnating, and if you want more than one, buy a small divider. If you want neons get an undergravel filter, they get sucked up by traditional filters. Mollies and guppies are also good.
For a fresh water tank you can have some mollies and platties together. Stay away from goldfish. For salt water you would want to stay away from small tanks.
Medium sized tanks can hold a couple gold fish, which only grow to their surrounding environment. A beta is good for tiny tanks, but aggressive so only get one.
neons, whiteclouds, guppies, danios are good fish for little tanks.Little tank is enough for these fishes.if you keep oxygen supply it will grow very nice.