THere are several pygmies that can be kept in a 10-15 g system, but most are fairly secretive and nocturnal. We often keep O. bocki and O. wolfi (which is the more difficult of the two). Both only live a few months as adults and probably have generation time of less than a year. O. mercatoris is a bit larger, but can surive in a well cycled 10 g. They are fairly easy to get from Florida gulf coast suppliers in the spring when they breed. We also keep Hapalochlaena lunulata and other small blue-ring species in small systems, but again they only live as adults for a month or two and, of course, they are dangerous.