This is an updated version of an article posted to Freshwater Aquariums forum
Have a wierd creature in your water? A bug in the tank? Tiny worms crawling on your aquarium glass? Use this handy guide to help you identify the organism.
(Smallest to largest.)
– Free floating green algae: this is what turns your water green. It looks like green smoke travelling through the water currents. Daphnia and cyclops feed on this. Green algae requires long hours of bright light and extra nitrates to live.
Danger: looks bad if you don’t want it. Good for feeding daphnia or cyclops or other infusoria.
– Cyclops: you can just barely see these whitish copepods that look like a capital Y, but they dart in the water in jerky motions. Females have 2 egg sacks which make them Y shaped. Males are just a white dot with a tail (tail is hard to see). I’ve seen a cyclops as large as 2mm. It’s a big female always with eggs. They are usually 1mm long.
Danger: Some people say they eat smaller daphnia and can crash a daphnia population.
– Daphnia: these are about the same size as a pinhead, or slightly smaller and are not white. They can be yellowish or greenish. They are round and swim slowly through the water, not jerkily like cyclops. About 1mm long.
Danger: Harmless. They make excellent fish food.
– Baby snails: newly hatched are a little smaller than pinheads or poppy seeds. They do not swim and only cling to any surface, like glass or plants.
Danger: Harmless. But adds more waste then your average fish.
– Hydra: these branched, soft-bodied things are like anemones. But they only have about 5 tentacles. They attach themselves to any material on the bottom, to logs, stocks, or plants. They reproduce by budding a copy of themselves on the side of the “stem”. Some are light yellow, some are green and use chlorophyll. They feed by snaring smaller foods in their stinging tentacles. 1-4mm long bodies excluding tentacles.
Danger: Potentially troublesome. They eat things like daphnia and cyclops and small fry.
– Nematodes: tiny, thin, HAIRLESS worms which swim by making S shapes back and forth. Not to be confused with mosquito or midge larva (below) which have hairs. Often pink or red colored. 1-5mm long.
Danger: Harmless to plants, inverts, fry and fish.
– Unknown worm: tiny, thin, hairless and whitish or yellow. Crawls on side of glass, seems to be searching for food. Might be a type of Nematode.
Danger: Harmless. Sometimes present when theres an excess of food in the aquarium.
– Tubifex: These are reddish thin worms which burrow their heads in the substrate and wave their tails in the water trying to get oxygen. The part you see is about 1-3mm. Their eggs could have come from a local pond, or from frozen tubifex. I don’t know if their eggs would survive freeze drying.
Danger: none. Provide food for some fish.
– Planaria: flat worms, like slugs, white to tan to brownish or gray. They all have an arrow shaped head with 2 eye spots.
Danger: Usually harmless, but Planaria will eat fish eggs.
– Mosquito larva: swim by making jerky S shapes, but these need air and float at the top of the water. If disturbed they flee to the bottom of the jar, then later come back up. These are like hairy worms with an insect head. These can be the same size as nematodes and confused with them, but they will get bigger than a nematode. These can be 4mm to 10mm long depending on age.
Danger: Harmless to all, but when they hatch into flying adults they can bite you at night.
– Midge larva: look just like mosquito larva but are red because they contain hemoglobin, same stuff as in mammal blood cells.
Danger: Harmless. Does not bite after hatching. Adult looks just like mosquito but with red abdomen.
– Medium thick wormy thing with black head: 4-5mm long. Looks a little like a grub with no legs, yellow-white body, black grublike head. It’s probably the larva of some type of water beetle.
– Snails: The link below will show you about aquarium apple snails and pond snails (ramshorn (planorbula) and apple-type pond snail (physa)). Some snails will eat your plants, but mainly they prefer algae.
Danger: They can potentially damage plants.
– Dragonfly nymphs: these are medium-fat bodied insect larva which later hatch into dragonflies.
Danger: Potentially damaging to small fish or small tadpoles. They will eat anything below 1″ in size including fish and fry.
– Water scorpion: these are larger insects which look like a walking stick. They hang around in water plants. Body is about 4″ long, with legs they are 7-8″ long. Yes, they have really long legs. They can bite you with their piercing mouth parts, be careful.
Danger: potentially harmful. Eats small fish (under 1″) and other insects in the water.
– Water beetles: generally, water beetles mean the underwater (not surface) swimming beetles which have piercing mouth parts and can grow up to 4″ long. They have a dark brown or black shell, their legs stick out very little when viewed from the top. THEY BITE. Also called “toe biters” as swimmers most often encounter them when they are stepped on.
Danger: will eat small fish (under 1″) and water insects. Can bite people. Handle with care. Handle by using thumb and finger to pinch on the sides of the shell so you will be well away from their mouth.
– http://www.microscopy-uk.org.uk/pond/x_index.html – Microscopic pond life with pics.
– http://www.hants.gov.uk/sparsholtschoolscentre/ponddatabase/virtual_pond_dip.htm – Pond life, macro and micro, with pics.
– http://www.cnas.smsu.edu/zooplankton/collage.htm 40+ pics of daphnia and copepods.