Rummy noses are a type of fish, known scientifically as Hemigrammus rhodostomus. They are native to the Amazon River Basin and have been kept in aquariums since the mid-1800s. While these fish tend to live up to five years, they can reach lengths of up to an inch and a half in adulthood. If you decide that you would like 15 gallons worth of rummy noses for your own tank, then you may want to consider keeping about 10-12 individual fish. Generally speaking, it is recommended that 1 gallon should be allocated per inch of full grown length when considering stocking levels for any given species – so if your rummy nose will grow into an average size adult at around 1 ½ inches, then 15 gallons could comfortably house 10-12 individuals without overcrowding them or risking water quality issues due to too many waste products being produced by the inhabitants.