If you want to improve your guppy aquarium, plants are a great next step. Plants compete with algae, depriving them of nutrients. They have their own requirements, like light and nutrients, and require their own care. However, some plant species can tolerate and even thrive under a beginner's care.
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Mosses are some of the most primitive plants. Java moss is an aquatic that is extremely tolerant of beginners' mistakes. This moss can tolerate almost any conditions you would see in a guppy aquarium. You can let the moss float freely or tie it down to a surface, where it will cling on and cover it like ivy. When it attaches to a surface, it forms a rich green lawn that covers the surface. It is sometimes sold in clumps called "moss balls."
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Java ferns look different from Java moss, but need very similar care. They have leaves and runners, but if tied down to a surface, they will attach themselves by the roots and grow just like Java moss. They easily tolerate a wide range of water conditions, temperatures and light conditions. They have a leafy look similar to that of several more difficult plants, and some recommend them as an easier alternative to plants like crypts or swords.
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Amazon swords grow well under good lighting. They also grow large, providing plenty of room for guppies to swim among the leaves. At the same time, their rapid growth and size means they eat up a lot of the nutrients that algae could use. They do need better lighting than Java ferns or Java moss, but require little else in the way of additional care.
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Aquarium hobbyists who keep watersprite generally keep it as a floating plant. You can plant it, but it will constantly try to reach toward the water's surface. Cold water damages it, but the tropical conditions in a guppy aquarium allow it to grow quickly. Guppies may nibble on it, but it grows back so quickly you'll never notice. Most importantly, baby guppies can hide in it until they are large enough to avoid getting eaten by other fish in the tank.