Several readers have sent emails asking which fish should be bred and how to tell when the fish are ready. In general you should select a pair that has the color and shape that most appeals to you. The male should always be larger than the female and you might want to choose one that has a great deal of energy with a vibrant color. The male is ready as soon as he starts making his bubble nest. The female will have a distended stomach culminating at the ovipositor. (white egg spot protruding from her abdomen) You will also see verticle stripes on the females flanks when she is ready to spawn. (If the stripes are horizontal this is a sign of stress) The female is capable of holding upwards of 500 eggs.
For more info see “How to Breed Beta Fish“.