These are some of the more common creatures you may find in your aquarium and a little info about them.
A healthy ecosystem contains many life forms and not all creatures found in your aquarium are a problem, in fact many are a sign of a well established and healthy tank/ ecosystem. So before you go starting a chemical war with them, take the time to know what it is you are looking at and if they are actually a problem.
Will cause problems for juvenile shrimp in aquariums
Damsel fly nymph (Odonata Zygoptera )
Size range 15 - 40mm
Damselfly Nymphs are easily recognized by their three long tail-like gills at the end of their slender bodies that grow between 16 and 33 mm. They have great vision, due to their large compound eyes. Damselfly Nymphs have extendable jaws that fold up under their head and legs close behind their head. Damselfly Nymphs are predators that feed mostly on other water insects, but they can also be cannibals. Larger species of Damselfly Nymphs can feed on small fish. They catch their food with a toothed lower lip, called a labium. When a small insect comes near, the nymph will shoot out its lower lip to grab its prey.
Dragon fly nymph (Odonata Anisoptera )
Size range 20 - 50mm
Dragonfly Nymphs are short and wide growing 18-49 mm long. They have six legs located near the head, wing pads, and internal gills. Dragonfly Nymphs are predators that feed mostly on other water insects, but they can also be cannibals. Larger species of Dragonfly Nymphs can feed on small fish. They catch their food with a toothed lower lip, called a labium. When a small insect comes near, the nymph will shoot out its lower lip to grab its prey.
Fresh water bristle worm (Annelida Polychaeta )
Size range 1- 100mm
Each Bristle worm has characteristic bristles that are found lining the exterior of their white or pink body. They typically grow between 1 and 100 mm, but have been known to grow up to 150 mm in length. Freshwater Bristle worms can be found in silt substrates and among debris or detritus in ponds, lakes, streams and rivers. They use their two tentacles to capture food or detritus floating by and transport it along cilia to the mouth.
May cause problems for juvenile shrimp if in high numbers
Planaria or flat worm (scientific name)
Size range 0.5 - 15mm
Planarians are usually between 3 to 15 mm with a triangular head that contains two eye spots. The colors vary from white, grey, brown, and black. The mouth is located on the underside of the worm halfway down towards its tail. Planarians suck up seed shrimp, clam shrimp, water fleas, and dead animals using a straw like appendage that extends from their stomaches.
Planarians can regenerate many of their body parts, including their heads!
Hydra (phylum Cnidaria)
Size range 0.2 - 15mm
Hydra has a tubular body up to 10mm long when extended, secured by a simple adhesive foot called the basal disc. At the free end of the body is a mouth opening surrounded by one to twelve thin, mobile tentacles. Each tentacle, or cnida (plural: cnidae), is clothed with highly specialized stinging cells called cnidocytes. Cnidocytes contain specialized structures called nematocysts, which look like miniature light bulbs with a coiled thread inside. At the narrow outer edge of the cnidocyte is a short trigger hair called a cnidocil. Upon contact with prey, the contents of the nematocyst are explosively discharged, firing a dart-like thread containing neurotoxins into whatever triggered the release which can paralyze the prey, especially if many hundreds of nematocysts are fired.Hydra mainly feed on small aquatic invertebrates such as Daphnia and Cyclops.
Not a problem
Nematodes (scientific name?)
Size range 0.01 - 10mm
Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. The total number of nematode species has been estimated to be about 1 million. Unlike cnidarians and flatworms, nematodes have tubular digestive systems with openings at both ends. Nematodes are slender worms, typically less than 2.5 mm (0.10 in) long. The smallest nematodes are microscopic, while free-living species can reach as much as 5 cm (2.0 in), and some parasitic species are larger still, reaching over a meter in length. The body is often ornamented with ridges, rings, bristles, or other distinctive structures.
Tubifex (tubificid annelids)
Size range 10 - 50mm
Tubifex worms feed on decaying organic matter, detritus, and vegetable matter, which is commonly available in sewage/stormwater drains. Tubifex worms are hermaphroditic: each individual has both male (testes) and female (ovaries) organs in the same animals. These minute reproductive organs are attached to the ventral side of the body wall in the celomic cavity. In mature specimens, the reproductive organs are clearly found on the ventral side of the body.
Seed Shrimp (Ostracoda)
Size range 0.2 - 30mm
The body of an ostracod is encased by two valves, superficially resembling the shell of a clam. A distinction is made between the valve (hard parts) and the body with its appendages (soft parts). In most ostracods, eggs are either laid directly into the water as plankton, or are attached to vegetation or the substratum. However, in some species, the eggs are brooded inside the shell, giving them a greater degree of protection. The eggs hatch into nauplius larvae, which already have a hard shell.
Daphnia (Branchiopoda Cladocera)
Size range 0.2 - 5mm
Daphnia are small, planktonic crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style. They live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers.
Cyclops, Copepods (Maxillopoda Cyclopoida)
Size range 0.5 - 5mm
Cyclops or water flea is one of the most common genera of freshwater copepods, comprising over 400 species. Cyclops individuals may range from 0.5 - 5 mm long and are clearly divided into two sections. The broadly oval front section comprises the head and the first five thoracic segments. The hind part is considerably slimmer and is made up of the sixth thoracic segment and the four legless pleonic segments. Two caudal appendages project from the rear. Although they may be difficult to observe, Cyclops has 5 pairs of legs. The long first antennae, 2 in number, are used by the males for gripping the females during mating. Afterwards, the female carries the eggs in two small sacs on her body. The larvae, or nauplii, are free-swimming and unsegmented.
Freshwater Limpet (scientific name)
Size range 0.5 - 5mm
Limpet is a common name for a number of different groups of sea snails and freshwater (aquatic gastropod molluscs). The common name is applied to those snails that have a simple shell which is broadly conical in shape, and either is not spirally coiled, or appears not to be coiled in the adult snail. In other words the shell of all limpets is shaped more or less like that of most true limpets.
Rotifers (scientific name)
Size range Small.
Rotifers have bilateral symmetry and a variety of different shapes. The body of a rotifer is divided into a head, trunk, and foot, and is typically somewhat cylindrical. Rotifers eat particulate organic detritus, dead bacteria, algae, and protozoans. They eat particles up to 10 micrometres in size. Like crustaceans, rotifers contribute to nutrient recycling. For this reason, they are used in fish tanks to help clean the water, to prevent clouds of waste matter. Rotifers affect the species composition of algae in ecosystems through their choice in grazing. Rotifers may be in competition with cladocera and copepods for phytoplanktonic food sources. Rotifers are dioecious and reproduce sexually or parthenogenetically. They are sexually, with the females always being larger than the males. In some species, this is relatively mild, but in others the female may be up to ten times the size of the male. In parthenogenetic species, males may be present only at certain times of the year, or absent altogether.
Most information collected from http://en.wikipedia.org/wiki/Main_Page
Also a great poster for identifying many of the water creatures