You liven up your fish tank with freshwater invertebrates. Many make great companions for guppies. Freshwater invertebrates can add interest to your tank. However, not all freshwater invertebrates will work in every aquarium. Some may harm fish like guppies. However, there are many safe options.
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You may or may not want snails in your guppy aquarium. If you keep plants, some species like ramshorn snails and apple snails may eat them. Other snails, like the Malaysian trumpet snail, are prized for their ability to scavenge excess food and "aerate" the gravel. Some people view all snails as pests and recommend avoiding them. For sure, once you make the decision to add snails to an aquarium, it is almost impossible to get rid of them.
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You may want to consider freshwater shrimp for your guppy aquarium. Freshwater shrimp have become more and more popular and have much to recommend them. The only caveat with shrimp in a guppy tank is to avoid the smallest shrimp. Guppies will eat very small shrimp. The ghost shrimp (also called the grass shrimp and the glass shrimp), the Amano shrimp and the bamboo shrimp work well with guppies and similar-size fish.
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Aquarium crabs are another interesting invertebrate. They are more predatory than shrimp, however, so choose only smaller crabs for a guppy aquarium. Be aware that crabs have aquarium requirements beyond those of guppies. Crabs usually require some dry areas to climb onto. Additionally, many freshwater crabs like red-clawed crabs require some salt added to their water. Fortunately, guppies are tolerant of some salt in their water. With these consideration in mind, you can keep guppies and crabs together.
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Like shrimp, crayfish have increased in popularity in recent years. New color varieties have recently emerged in pet shops, such as the electric blue crayfish. But you should not keep crayfish with guppies. Crayfish will eat guppies if they can catch them. If you want to keep crayfish, you should keep them in a tank of at least 30 gallons with fish that are large enough that the crayfish cannot eat them.