Q. I have a 10-gallon aquarium with seven guppies and two small mollies. Two of the guppies are males. The guppies are of various colors and strains. The guppies do not seem to be breeding, and I'm wondering if it is because the females are smaller than they should be or if the tank is overcrowded. Also, the local pet store where I shop keeps male and female guppies of the same strain in the same tank. Shouldn't they be kept separately?
A. Your aquarium is not exactly overcrowded, but you don't really supply me with enough information to determine whether there is a problem that is preventing the guppies from breeding. Small size in females can be caused by many things: age, genetic stock, feeding, tank conditions and so on.
Fish in pet shops are often stocked under less than ideal hobbyist conditions because of a lack of space and a desire to provide as many different types of fish as customers want to buy. Housing male and female guppies together provides for better utilization of space and makes it easier to sell the fish in pairs. Keeping the strains separated from each other ensures that the stock does not become mixed. The only time it is necessary to keep males separate from females is when it is desirable to have virgin females for controlled matings, which is the job of the hobbyist, not the dealer.