The Growth of a Guppy

Guppy babies are fully formed when they’re born, but are quite small, with an average size of about 0.25 inch. Other fish in the tank, including their parents, see the babies as food and will eat them if they can catch them -- so you keep babies in a tank by themselves until they are at least an inch long. They will grow to full size in about six months, which is 1.5 inches long for males, and 2.5 inches for females, not counting the tail.

Temperature can have a significant impact on how fast your guppy babies grow. Babies kept in a warm tank, about 80 degrees, will grow much faster than those kept in a tank in which the temperature hovers in the mid-70s. This is because the warmer water gives a boost to guppy metabolism; everything speeds up, including their development. If you leave them in 80 degree water as adults, though, they will continue to have accelerated metabolisms and will typically die more quickly than if they were kept in water a few degrees cooler.

A good diet helps guppies to develop as quickly as possible. These fish are omnivores, so they need both animal and vegetable matter. Feed them four to eight times a day, cutting back to once or twice a day as they mature. Baby brine shrimp are easy to hatch at home and make an excellent food for young guppies. Other good foods are microworms, bloodworm, spinach and algae. Crumble commercial guppy flakes for them when the fish are tiny, and continue using these as a part of their daily diet to make sure your guppies get the variety they need.

Water quality has a lot to do with how well and how quickly your baby guppies develop. Fish waste creates nitrates, and when nitrates build up (indicating too many fish in the space) the guppies’ growth slows down. To keep the babies growing, change out about a third of the water each week. This removes the nitrates and gives a boost to guppy growth. If you’re ambitious you can replace about 10 percent of the water every day, as some breeders do, but your guppies will do fine with weekly changes.