Guppies can exist in a wide tempeature range and are amazingly tolerant of slow changes in temperature. That being said, guppies prefer tempeature between 70 and 80F. Being cold-blooded animals their growth and maturation rate can be influenced by temperature. For example, if you take a drop of babies and grow half at 70 and half at 80 you will see big differences in growth. However, guppies grown at higher temperature do not live as long. Tempearture is, however, one thing you can manipulate to get guppies to be at the right stage of growth for a particular show.
To adjust and maintain temperature each tank needs to have a heater. If you have a large number of tanks this is impractical and it is much better to have a guppy room zoned on its own thermostat. Since the room will stratify - warmer near the ceiling, colder near the floor, you may want to use a fan to equalize temperature. Some breeders take advantage of the unequal tempeature by putting fish they want to grow faster in tanks closer to the ceiling and putting their show fish in colder water in tanks near the floor so that they last longer.
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