The guppy runs a close second to the goldfish in the category of recognizable aquarium fish. This little fish has a lot to recommend it, both for people setting up their first tank and seasoned hobbyists. Guppies have bright colors, adapt well to a variety of water conditions and get along well with other fish. However, it does take some attention to detail when picking out a healthy guppy to avoid introducing disease into your tank.
Watch how your guppy behaves in the pet shop. Make sure the guppy is actively swimming around. A guppy hanging out at the bottom of a tank could be ill. Also, look for guppies that swim to the top of the tank and "beg" for food. This doesn't mean they're underfed; guppies will beg even with full stomachs. This behavior means they're fed often enough that they associate humans with food.
Pay special attention to the guppies' fins. Guppies can spread out their fin rays, making their fins fan out. You want guppies with flared-out fins, not fins clamped against their bodies. Also look for signs of infection like white spots, dots or fuzzy patches. Make sure there are no rips, holes or tears in the fins. Avoid fish that have a ragged edge to their fins, as this is a sign of fin rot.
At the same time, look closely at the guppy's head. The eyes should be clear and not have a cloudy look. Additionally, make sure the eyes aren't protruding. Bulged-out eyes can be a sign of a condition called "popeye" caused by infections behind the eyeball. Also watch the gills. Avoid fish that are breathing rapidly, flapping their gills more than once a second. Lastly, take a close look at the mouth to check for ulcers.
You also need to look at the guppy's body for signs of illness. Avoid purchasing guppies that have unusually curved or deformed spines. Look at the belly. It should be rounded but not swollen, except in pregnant female guppies. Do not purchase fish with sunken bellies or ones that have a "pinched" look where the body meets the head. This can be a sign of a chronically undernourished or ill fish. Just like the fins, make sure the body doesn't have any white spots, patches or fuzz, which can be bacteria and protozoan parasites.
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